Fix Jenkins job that's failing for Security Guide.

There's no copy of the Security Guide in the
stable/grizzly branch of openstack-manuals so
setting to just build from master branch in zuul.

Fix bug 1197632

Change-Id: I694d23ed96b43aaa9b12406f217f74975b1b06ac
Reviewed-on: https://review.openstack.org/36549
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Tom Fifield <tom@openstack.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins