Replace inner-/external computes with a dvr mode variable

Bug https://bugs.launchpad.net/kolla-ansible/+bug/1722026 and fix https://review.openstack.org/#/c/510327 introduced two compute groups [inner-compute] and [external-compute] which replace good known [compute]. This was done just to differentiate between dvr modes if dvr is desired. The groups are not compatible with older releases and also are not easy to understand by users.

Instead of new groups it is suggested to bring in a new property 'compute_dvr_mode' that will be set to 'dvr' by default, meaning old dvr behavior with external interface / traffic from compute nodes. Other mode will be 'dvr_no_external'. If this flag is set to internal-only then we also should not require internal interface / bridge setup on compute hosts.

If operators really want computes with different dvr modes then it still is possible via per host or per
group variables.