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.

Closes-Bug: 1636362
Change-Id: Ied9449896dddbbfc050de7da83a8e447062e2caa

openstack/murano,openstack/murano