[releng] Introduce support for Eclipse Kepler

Kepler has been released on June 26, 2013.
- use it as a baseline target platform
- run integration tests against 3 releases: Kepler (June 26, 2013), Juno (June 27, 2012) and Indigo (June 22, 2011)
- pre-Kepler M2E bundles has been removed from main M2E P2 repository, use new locations in Juno- and Indigo-based TP