Fix run-docs as a gate job.

If run-docs is running as a gate script, ZUUL_REFNAME is not set.
We also don't care where the docs end up so do nothing (treat as
master).

Change-Id: I8f5a7ba4c6ab714765871c60b5e587fcb4dece41
Reviewed-on: https://review.openstack.org/31889
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins