Mods to "Push" goal

From feedback from Wouter:

- modified push goal to reuse stored built image info and identified
which images to push by an additional property
- updated documentation

Main functional bits here:

- added <push>true/false</push> to BuildImageConfiguration
- updated BuiltImageInfo to retain name-and-tag + push
- stripped out <imageIds> from push mojo and instead made it look back
to built image info
- disables removal of images on stop (i.e. keep = false) if those images
were flagged for push

On the last bit, I tried to then remove those images after the push
instead, however, Docker gave me back 409-Conflict implying the push was
somewhat asynchronous and therefore some delay required for removal. For
now, am assuming all pushed images are implicitly "kept" which seems a
reasonable assumption. (Having pushed it, do you really want to pull it
again?)