[API] resolving nicknames no longer need a SarosNet object argument

As preparation to remove the session object from the User class
resolving a nickname does no longer needs a SarosNet object.

Added a hack so that the RosterUtils initializes itself with
the correct SarosNet object.

Change-Id: Ia6700d451869fb6b3e952bdfb1d4ba4e4a087cab
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/910
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>