Add requirements.txt to docs reqs
Add requirements.txt to docs reqs

senlin-dashboard doc generated the module reference,
so requirements.txt should be here to apply upper-constraints.

Change-Id: Ic1329872733397842bba0187440557df37c32826

openstack/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,stackforge/senlin-dashboard,stackforge/senlin-dashboard