Update Gerrit docs for CLA and group changes.

* doc/source/gerrit.rst: Changes to the CLA process and group
management have removed the need for Launchpad synchronization.

* doc/source/stackforge.rst: Rewrite the core group section to refer
to new Gerrit-specific workflow instead of the old Launchpad method.

* doc/source/systems.rst: Remove recommendation for adding core
groups to Launchpad since those are a Gerrit-only concept now (bugs
and drivers groups remain relevant to LP however).

Change-Id: I2532b93a43821848bb48d12fcaf3f1b0b07dd207
Reviewed-on: https://review.openstack.org/17868
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins