Online update mode for compute nodes

Addition of a basic online update mode for compute nodes, that
when -e online_upgrade=True is passed as an argument on the ansible
command line, the update_cloud.yml playbook will enter the new logic
and attempt to update the nova-compute instance without
a reboot.

Change-Id: I566a9d465d1cd1a7e6f2d6795ccf83d0c2dcbbcf