[INTERNAL] replaced VCSAdapter with VCSProvider interface

Far from perfect but ensures that the file list classes can be moved to
the core. 

TODO: Add a ProviderManager to lookup providers.

Change-Id: I9454089b3a013c423373c80a9392ade41590f77e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1641
Tested-by: Jenkins-Core QA
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>