Add Ability to Reload Environments

Description of Issue

I run my masters in containers with the salt-master being the command that I run. From time to time I have need to update my file_roots setting. This seems to require a full restart of the master daemon for the environments to work correctly. I can see the environment on the master with salt-run fileserver.envs without restarting the master, but the minion never seems to see the new environment until I restart the daemon.

The feature request is to have the master updated to allow for refreshing the environments configuration.

Setup

Add a new file_root configuration while Salt master is running.

Run salt-run fileserver.envs to show the new environment appears on the master.
Try running salt-call cp.list_master saltenv=new.environment to show the minion does not see the new environment.

# After adding v1.3 file_root to /etc/salt/master.d/fileserver.conf
[root@master1 v1.3]# salt-run fileserver.envs
- base
- v1.2
- v1.3
[root@master1 v1.3]# salt master1 cp.list_master saltenv=v1.3

master1:
    ----------
# After restarting the master daemon
[root@master1 v1.3]# salt master1 cp.list_master saltenv=v1.3
master1:
    - test12.sls
    - testnew.sls
    - top.sls

Versions Report

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: 1.6.0
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.7.2
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.5 (default, Aug  7 2019, 00:51:29)
   python-gnupg: Not Installed
         PyYAML: 3.11
          PyZMQ: 15.3.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4

System Versions:
           dist: centos 7.6.1810 Core
         locale: UTF-8
        machine: x86_64
        release: 5.4.13-201.fc31.x86_64
         system: Linux
        version: CentOS Linux 7.6.1810 Core

Start by tracing the master fileserver behavior exercised by salt-run fileserver.envs and salt-call cp.list_master, using the file_roots setting in /etc/salt/master.d/fileserver.conf as the configuration entry point. Reproduce the setup with a newly added environment, then verify that the minion lists it without restarting the master daemon.