Rework neutron system host management to operate on agent states
rather than an extended host entity, as it was agreed with the
neutron team that a new host level entity was not desired in the
neutron core.
DHCP and L3 agents on hosts with openstack_compute labels are
now managed by the VIM.
Story: 2003857
Task: 26663
Change-Id: I441fcf3c186f68d17abafe337af71d0caf9c40da
Signed-off-by: Kevin Smith <kevin.smith@windriver.com>