chef-server-ctl backup fails due to missing rsync

Bug:
running chef-server-ctl on a fresh install of CentOS provided by Azure, the backup command throws an error due to missing rsync

Expectation:
backup should work without external dependencies or it should notify the user of missing dependencies

Details:

Starting Chef Server backup
WARNING:  Offline backup mode must stop your Chef server before continuing.  You can skip this message by passing a "--yes" argument. Do you wish to proceed? (y/N):
y
Bringing down the Chef Server
ok: down: bookshelf: 0s, normally up
ok: down: nginx: 1s, normally up
ok: down: oc_bifrost: 0s, normally up
ok: down: oc_id: 0s, normally up
ok: down: opscode-chef-mover: 517266s, normally up
ok: down: opscode-erchef: 0s, normally up
ok: down: opscode-expander: 0s, normally up
ok: down: opscode-expander-reindexer: 0s, normally up
ok: down: opscode-reporting: 1s, normally up
ok: down: opscode-solr4: 0s, normally up
ok: down: rabbitmq: 0s, normally up
ok: down: redis_lb: 1s, normally up
Dumping Postgresql database to /tmp/chef_backup20151201-5526-1k70r5a/chef_backup-2015-12-01-21-53-38.sql
could not change directory to "/root/temp"
ok: down: postgresql: 0s, normally up
Writing backup manifest
Creating backup tarball
tar: Removing leading `/' from member names
Bringing up the Chef Server
ok: run: bookshelf: (pid 5745) 0s
ok: run: nginx: (pid 5799) 0s
ok: run: oc_bifrost: (pid 5820) 1s
ok: run: oc_id: (pid 5874) 1s
ok: run: opscode-chef-mover: (pid 5890) 0s
ok: run: opscode-erchef: (pid 5939) 0s
ok: run: opscode-expander: (pid 5966) 0s
ok: run: opscode-expander-reindexer: (pid 5998) 1s
ok: run: opscode-reporting: (pid 6040) 0s
ok: run: opscode-solr4: (pid 6124) 0s
ok: run: postgresql: (pid 6172) 1s
ok: run: rabbitmq: (pid 6229) 1s
ok: run: redis_lb: (pid 6501) 0s
Exporting tarball to /var/opt/chef-backup
ERROR: Something wen't terribly wrong, aborting backup
ERROR: No such file or directory - rsync
Cleaning up /tmp/chef_backup20151201-5526-1k70r5a
Bringing up the Chef Server
ok: run: bookshelf: (pid 5745) 62s
ok: run: nginx: (pid 5799) 61s
ok: run: oc_bifrost: (pid 5820) 62s
ok: run: oc_id: (pid 5874) 61s
ok: run: opscode-chef-mover: (pid 5890) 60s
ok: run: opscode-erchef: (pid 5939) 60s
ok: run: opscode-expander: (pid 5966) 59s
ok: run: opscode-expander-reindexer: (pid 5998) 59s
ok: run: opscode-reporting: (pid 6040) 58s
ok: run: opscode-solr4: (pid 6124) 57s
ok: run: postgresql: (pid 6172) 57s
ok: run: rabbitmq: (pid 6229) 57s
ok: run: redis_lb: (pid 6501) 56s
/opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout/unix.rb:338:in `exec': No such file or directory - rsync (Errno::ENOENT)
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout/unix.rb:338:in `block in fork_subprocess'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout/unix.rb:316:in `fork'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout/unix.rb:316:in `fork_subprocess'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout/unix.rb:93:in `run_command'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/mixlib-shellout-2.2.0.rc.0/lib/mixlib/shellout.rb:259:in `run_command'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/chef_backup-0.0.1.dev.4/lib/chef_backup/helpers.rb:47:in `shell_out'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/chef_backup-0.0.1.dev.4/lib/chef_backup/strategy/backup/tar.rb:173:in `export_tarball'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/chef_backup-0.0.1.dev.4/lib/chef_backup/strategy/backup/tar.rb:145:in `backup'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/chef_backup-0.0.1.dev.4/lib/chef_backup/runner.rb:28:in `backup'
        from (eval):30:in `block in load_file'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/omnibus-ctl-0.4.2/lib/omnibus-ctl.rb:190:in `call'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/omnibus-ctl-0.4.2/lib/omnibus-ctl.rb:190:in `block in add_command_under_category'
        from /opt/opscode/embedded/lib/ruby/gems/2.1.0/gems/omnibus-ctl-0.4.2/lib/omnibus-ctl.rb:661:in `run'
        from /opt/opscode/embedded/service/omnibus-ctl/chef-server-ctl:192:in `<main>'

workaround:
yum install rsync
chef-server-ctl backup -y

reference: https://github.com/chef/chef/issues/4217

Start in chef_backup/strategy/backup/tar.rb at export_tarball and trace the shell_out helper in chef_backup/helpers.rb. Reproduce chef-server-ctl backup on a fresh CentOS installation without rsync, then verify the backup either completes without that external dependency or reports the missing dependency clearly.