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: I153c8c7ce89c7f8ed498d22cf24459054ac9b8d4
Closes-Bug: #1552251

openstack/python-mistralclient,StackStorm/python-mistralclient,StackStorm/python-mistralclient,openstack/python-mistralclient