Add docker checks to consul backend

[threetee]

Fixes #414. Also adds check string token substitutions for external host/port.

[djlindstr]

Duplicate of #332?

[aaronhurt]

I'd be willing to take this over and submit a new PR if @threetee is no longer available.

[aaronhurt]

Okay I submitted #510 that includes this PR with a rebase from current master and a bit of touchup to the documentation. I also included credit to @threetee in the commit.

[aaronhurt]

I'm not usually one to claim other peoples PRs but I would really like this feature implemented and the original author does not look to be active based on his profile.

[promorphus]

Bump, I could really use this, and things seem to be stagnating.