[INTERNAL] changed provider inheritance

Added a new abstract base class ProjectNegotiationExtension
that should be used as superclass when adding new extensions
that are used during the project negotiation.

Change-Id: If6d85bbe959ae46b073da2c3eb5f2906ac1b5f6c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/926
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>