[BUG] Include directive in sshconfig results in parse error when using salt-ssh / sshconfig roster

Description
Using the sshconfig roster with salt-ssh will fail with a parse error if the sshconfig uses the "Include" directive

Setup

• salt 3004.1 running on a Centos 7 VM

Steps to Reproduce the behavior

• with an "Include" directive in the /etc/.ssh/config:

Include included_ssh_config_directory/*

# rest of the host blocks here

• run any salt-ssh command with the sshconfig roster

Traceback (most recent call last):
  File "/usr/bin/salt-ssh", line 11, in <module>
    load_entry_point('salt==3004.1', 'console_scripts', 'salt-ssh')()
  File "/usr/lib/python3.6/site-packages/salt/scripts.py", line 459, in salt_ssh
    client.run()
  File "/usr/lib/python3.6/site-packages/salt/cli/ssh.py", line 21, in run
    ssh = salt.client.ssh.SSH(self.config)
  File "/usr/lib/python3.6/site-packages/salt/client/ssh/__init__.py", line 240, in __init__
    self.targets = self.roster.targets(self.opts["tgt"], self.tgt_type)
  File "/usr/lib/python3.6/site-packages/salt/roster/__init__.py", line 100, in targets
    targets.update(self.rosters[f_str](tgt, tgt_type))
  File "/usr/lib/python3.6/site-packages/salt/loader/lazy.py", line 149, in __call__
    return self.loader.run(run_func, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/salt/loader/lazy.py", line 1201, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/contextvars/__init__.py", line 38, in run
    return callable(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/salt/loader/lazy.py", line 1216, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/salt/roster/sshconfig.py", line 99, in targets
    all_minions = parse_ssh_config([line.rstrip() for line in fp])
  File "/usr/lib/python3.6/site-packages/salt/roster/sshconfig.py", line 56, in parse_ssh_config
    hosts[-1].append(line)
IndexError: list index out of range

The code appears to assume that all lines are either empty, start with a comment, or start with "Host", and a line that starts with "Include" would break that assumption.

Expected behavior

• ideally I'd like for the ssh config parsing to collect all the files listed in the include directives and include them alongside all the other host entries
  • this is similar to what paramiko did in early 2018: https://github.com/paramiko/paramiko/pull/872
  • in the absolute worst case, ignoring include directives and not bombing out is preferred to the current behavior

Versions Report

Salt Version:
          Salt: 3004.1
 
Dependency Versions:
          cffi: Not Installed
      cherrypy: unknown
      dateutil: 2.4.2
     docker-py: Not Installed
         gitdb: 0.6.4
     gitpython: 1.0.1
        Jinja2: 2.11.1
       libgit2: Not Installed
      M2Crypto: 0.35.2
          Mako: Not Installed
       msgpack: 0.6.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: Not Installed
      pycrypto: 2.6.1
  pycryptodome: Not Installed
        pygit2: Not Installed
        Python: 3.6.8 (default, Mar 30 2020, 17:04:00)
  python-gnupg: Not Installed
        PyYAML: 3.13
         PyZMQ: 17.0.0
         smmap: 0.9.0
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.1.4
 
System Versions:
          dist: centos 7 Core
        locale: UTF-8
       machine: x86_64
       release: 4.4.131-1.el7.centos.x86_64
        system: Linux
       version: CentOS Linux 7 Core

Start with salt/roster/sshconfig.py, especially parse_ssh_config and the sshconfig roster targets entry point. Reproduce the failure using an Include directive and salt-ssh with the sshconfig roster. Done means the roster no longer raises an IndexError and handles the directive according to the issue's expected behavior.