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: Ic8912e95e8e298fede1d94c20d3167d1f206df1e

jimbobhickville/taskflow,openstack/taskflow,openstack/taskflow,jimbobhickville/taskflow