On systemd hosts, notify systemd when master is started

**This looks to be working now for @gtmanfred, but I'm still not 100% sure if this is the best solution. Needs further scrutiny.**

Per a discussion with @gtmanfred in #salt, it would be useful to have the master notify the systemd daemon once it finishes starting up, thus allowing the systemd unit to be changed from using ``type=simple`` to ``type=notify``.

@thatch45, this is what I came up with, please let me know if I am wrong here.