Boot should validate Maven central deployment

Hello boot folks!

An idea I had while trying to deploy to Maven Central today is that we could probably detect the target repository, which usually comes in a `:repo-map` like:

:repo-map {:url "https://oss.sonatype.org/service/local/staging/deploy/maven2"
:username username
:password password}}

...and if we detect that it is the staging Maven Central repo, we make sure that the pom has all the [requirements](http://central.sonatype.org/pages/requirements.html).

Start by tracing deployment handling around the :repo-map and pom generation. Review the Maven Central requirements linked in the issue and determine which checks apply when the target URL is the Sonatype staging repository. Done means deployment to that repository validates the pom requirements before proceeding.

Written by the indexing model from the issue text.