Add regex start/end to gate-docs metajob.

Change-Id: I6d75af72fc729ceb8e735e10cfa02a5a1b122e25
Reviewed-on: https://review.openstack.org/26631
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins