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: Ie6477df7f65e5035b1b03b51f698972ce9dd9aaa
Closes-Bug: #1552251

openstack/python-saharaclient