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: I992f38ba935536c59217e22d4071f40a69090fc2
Closes-Bug: #1552251

ityaptin/ceilometer,ityaptin/ceilometer,openstack/ceilometer,openstack/ceilometer