Update target definitions to eclipse 2018-12

- Update nightly target to 2018-12
- Update release target to 2018-12
- Remove oxygen target
- Create previous-release target (2018-09)
- Update maven configuration
- Remove EMF Compare from p2 repository

Change-Id: Iecd0f3bd7b40ebe80637048431afa60461eec209
Signed-off-by: Tobias Ortmayr <tortmayr@eclipsesource.com>
17 files changed