Create gate for api-site

Create a new job-group that will be used by operations-guide, api-site,
openstack-manuals and perhaps API project repositories.
Use job-group for gating of api-site.

Setup template openstack-doc-gate in Zuul.
Setup api-site gates using new template openstack-doc-gate.
The niceness gate is non-voting for now.

Change-Id: I48b95fe17b79150c64f73e742a2e6e9d06050516
Merge "Create gate for api-site"