Deploy limiter plugin

[chrisarcand]

Since we ask people to refrain from deploys while testing failovers and over maintenance fairly often, we could write something that makes a request on an endpoint somewhere to see if you're allowed to deploy to an app/environment before making the call to OpsWorks. If not, return a reason message (set by the ops team).

I stress that I'd write this as a plugin, not as part of the core gem here.