Disabled (but running) systemd service is not restarted with mod_watch

Description of Issue/Question

When a systemd service is created, started, but not enabled, salt does not restart the service on mod_watch:

----------
          ID: /etc/zabbix/zabbix_agentd.conf
    Function: file.managed
      Result: True
     Comment: File /etc/zabbix/zabbix_agentd.conf updated
     Started: 10:14:52.621831
    Duration: 4298.499 ms
     Changes:
              ----------
              diff:
                  ---
                  <snip>
----------
          ID: zabbix-agent-service
    Function: service.running
        Name: zabbix-agent
      Result: True
     Comment: Service zabbix-agent has been enabled, and is in the desired state
     Started: 10:14:56.922787
    Duration: 1427.353 ms
     Changes:
              ----------
              zabbix-agent:
                  True

Some packages (zabbix, especifically), don't enable their services when installed, so Salt only enables it, instead of enabling+restarting.

This issue is discussed more thoroughly at this PR:
https://github.com/saltstack-formulas/zabbix-formula/pull/77

Setup

Formula: https://github.com/edusperoni/zabbix-formula
states:

• zabbix-agent.repo
• zabbix-agent.conf

Steps to Reproduce Issue

On a machine that has never installed zabbix-agent, install it using the formula above.

On a machine that has zabbix-agent installed, run systemctl disable zabbix-agent && apt-get purge zabbix-agent -y and then apply the state.

Versions Report

Salt Version:
           Salt: 2018.3.0

Dependency Versions:
           cffi: 0.8.6
       cherrypy: 3.5.0
       dateutil: 2.2
      docker-py: Not Installed
          gitdb: 0.5.4
      gitpython: 0.3.2 RC1
          ioflo: Not Installed
         Jinja2: 2.9.4
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.2
   mysql-python: 1.2.3
      pycparser: 2.10
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.9 (default, Jun 29 2016, 13:08:31)
   python-gnupg: Not Installed
         PyYAML: 3.11
          PyZMQ: 14.4.0
           RAET: Not Installed
          smmap: 0.8.2
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.0.5

System Versions:
           dist: debian 8.10
         locale: UTF-8
        machine: x86_64
        release: 3.16.0-4-amd64
         system: Linux
        version: debian 8.10

Start at Salt's service.running state and its mod_watch handling for systemd services, then reproduce the zabbix-agent scenario described in the issue. Done means a running but disabled service is restarted when its watched file changes, while the existing enablement behavior remains correct; add or update coverage for this case.