create apt_get_update to try to work around broken mirrors
create apt_get_update to try to work around broken mirrors

Ubuntu's apt mirroring mechanism produces inconsistent mirrors pretty
regularly. The devstack-gate apt-get update model seems to have been
more effective getting past this than what we did in devstack. Adopt
that method for our updates.

Change-Id: I97c7896ef38b275aacb4f933fc849acee1bab858

varunarya10/devstack,samgoon/devstack,dtroyer/devstack,group-policy/devstack,dirkmueller/devstack,noironetworks/devstack,avvocatodemarchis/devstack,Mahito/devstack,nawawi/openstack,noironetworks/devstack,samgoon/devstack,mssumanth/devstack,bigswitch/devstack,dtroyer/devstack,bigswitch/devstack,bswartz/devstack,nawawi/openstack,group-policy/devstack,varunarya10/devstack,richliu/devstack,avvocatodemarchis/devstack,openstack-dev/devstack,richliu/devstack,Mahito/devstack,thomasem/devstack,dirkmueller/devstack,openstack-dev/devstack,thomasem/devstack,mssumanth/devstack,bswartz/devstack