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: I7b8e5e23bc9dc439bc29fcf791fc44d0c82cb8ab
Closes-Bug: #1552251

openstack/mistral-specs