systemd module misses some files when checking for new unit files

Description of Issue/Question

The systemd module seems to only check for new unit files in /etc/systemd/system. This causes issues with new managed unit files that are placed in other valid locations for systemd. service.running and other related states fail to detect the new unit files and reload systemd before starting/stopping or otherwise managing services thus causing the entire state to fail.

There are multiple valid paths for unit files. Personally I was using /usr/local/lib/systemd/system/ (which seems to be ubuntu specific?) but the systemd.unit man page lists a bunch for both user and system modes. The three main system mode ones seem to be:

• /etc/systemd/system
• /run/systemd/system
• /lib/systemd/system

The relevant code seems to be in _untracked_custom_unit_found() in modules/systemd.py:
https://github.com/saltstack/salt/blob/develop/salt/modules/systemd.py#L375

Setup

service-test:
  file.managed:
    - name: /usr/local/lib/systemd/system/blah.service
    - source: salt://blah.service
  service.running:
    - enable: true
    - require:
      - file: service-test

Versions Report

Salt Version:
           Salt: 2018.3.3
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: 2.0.3
      gitpython: 2.1.8
          ioflo: Not Installed
         Jinja2: 2.10
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: 1.0.7
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.15rc1 (default, Nov 12 2018, 14:31:15)
   python-gnupg: 0.4.1
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: 2.0.3
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.2.5
 
System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-43-generic
         system: Linux
        version: Ubuntu 18.04 bionic
 

Start in salt/modules/systemd.py at _untracked_custom_unit_found(), then compare its checks with the systemd.unit paths listed in the issue. Reproduce the service-test setup using /usr/local/lib/systemd/system/blah.service and verify service.running detects the new unit and reloads systemd before managing it.