Disable concurrent builds for pre-release-gerrit

Disable concurrent builds for pre-release-gerrit

As pre-release-gerrit job updates metadata in OBS concurrent builds
cause raise conditions for group submissions.
If group submission(multiple git trees tagged with the same submit/ tag)
is used several pre-release-gerrit builds can update meta in OBS projects
at the same time. This causes breakages like this:
Traceback (most recent call last):
  File "job_pre_release_gerrit.py", line 154, in <module>
    sys.exit(main(os.path.basename(sys.argv[0])))
  File "job_pre_release_gerrit.py", line 148, in main
    build, obs_target_prj, project)
  File "job_pre_release_gerrit.py", line 66, in create
    build.unlink_project(obs_project)
  File "common/buildservice.py", line 516, in unlink_project
    raise ObsError('project: %s has not links' %(project))

This happens whtn one job-pre-release-gerrit build disables the linking of
prerelease project and another build tries to also disable it.

Fixes: #1845

Change-Id: I50e1e7fdf6a9d6afa8e03b053f1135aa96553648
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>