Switch jobs from Ubuntu Quantal to Precise slaves.

All jobs previously using a quantal node label are now using a
precise node label instead. Documentation has also been updated to
reflect the change in preferred release/image name.

Change-Id: I636416a9d9dc6714cd9751e2210882db2cbf786e
Reviewed-on: https://review.openstack.org/28861
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins