Don't set html_last_updated_fmt without git
Don't set html_last_updated_fmt without git

Using a timestamp as value for "html_last_updated_fmt" isn't good
for the reproducible builds initiative from different downstream
distributions because the build result always differs due to the
timestamp in the documentation.
Also when not setting "html_last_updated_fmt", the value can be set
via command line when running "sphinx-build -D".

Change-Id: Ia23451e6b53f6f83dd22e069ef42a4a067ec5881

openstack/osprofiler,stackforge/osprofiler,openstack/osprofiler,stackforge/osprofiler,openstack/osprofiler,stackforge/osprofiler