Rename [:ha][:enabled] attribute to [:ha][:server][:enabled]

This will allow us to introduce :ha][:l3][:enabled] later to differentiate
between the HA setups of neutron-server and l3-agent.
Skeleton code for l3-agent ha

This introduces the HA enablement pieces for the l3-agent. Currently this l3_ha
recipe is still a dummy and does nothing useful.
Implement l3_ha recipe

Create pacemaker primitive, group and clone for the various neutron agents.
Setup the neutron-ha-tool to perform automatic migrations of offline
routers.
l3_ha: Don't start l3-agent when using "vmware" networking
Include linux-bridge/ovs-agent into the l3 HA group