Mirage2 POM cleanup/refactoring, related to DS-2052

This PR simply cleans up the new Mirage2 POMs (added in DS-2052) to make them more in line with our other POMs. Namely it:

* Avoids duplication of plugin versions in several POMs (moving some versions to our Parent POM). Also updated those versions to the latest available (which all seem to work fine).
* Changes build dependency version numbers (for Node.js, etc) into POM properties, to make them easier to find and tweak. Also updated some of those versions to the latest available (which all seem to work fine).
* Removes / cleans up some bad spacing/tabbing, and also cleans up some empty POM tags which were not doing anything.

Essentially, this PR is just a refactoring of the POMs...it doesn't change what they do, just streamlines it.

Assuming Travis doesn't complain, I'll get this merged soon. It's just cleanup which is necessary but not controversial.