un-hardcode vault token monitoring in watcher

[eikenb]

The watcher has a hardcoded field for monitoring the vault-token as part of its process. This means for testing the vault-token you need to spin up a watcher with that config.

Seems like it would be better to integrate it without the hardcoding. In a way that would be easier to test.

I've refactored this already in consul-template and should use that as a guide to convert here after the non-template notifiers work.