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: I9a6aba18d40877f24c31d0b44cae4aca4b064655
Closes-Bug: #1552251

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral