Fix doc build if git is absent
Fix doc build if git is absent

When building packages if git is absent, then we should not set
html_last_updated_fmt. It can still be set via the -D switch
when building with sphinx-build.

Change-Id: Iea0fb01314e1c4a66a55841df07b9bdaf10153a6
Closes-Bug: #1552251

mahak/keystone,rajalokan/keystone,rajalokan/keystone,ilay09/keystone,openstack/keystone,cernops/keystone,openstack/keystone,ilay09/keystone,rajalokan/keystone,ilay09/keystone,mahak/keystone,openstack/keystone,cernops/keystone,mahak/keystone