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: I905736fc5f998385128b306c75bc8191b2797dc6
Closes-Bug: #1552251

yeming233/horizon,wolverineav/horizon,Mirantis/mos-horizon,sandvine/horizon,yeming233/horizon,coreycb/horizon,wolverineav/horizon,NeCTAR-RC/horizon,wolverineav/horizon,ChameleonCloud/horizon,noironetworks/horizon,openstack/horizon,Mirantis/mos-horizon,coreycb/horizon,BiznetGIO/horizon,bigswitch/horizon,yeming233/horizon,Mirantis/mos-horizon,bac/horizon,openstack/horizon,sandvine/horizon,ChameleonCloud/horizon,bigswitch/horizon,noironetworks/horizon,NeCTAR-RC/horizon,openstack/horizon,wolverineav/horizon,noironetworks/horizon,BiznetGIO/horizon,bigswitch/horizon,bac/horizon,coreycb/horizon,sandvine/horizon,bac/horizon,Mirantis/mos-horizon,bigswitch/horizon,coreycb/horizon,yeming233/horizon,NeCTAR-RC/horizon,noironetworks/horizon,BiznetGIO/horizon,openstack/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,sandvine/horizon,bac/horizon