Use ucarp for network HA.

Configured thusly:

`undercloud_floating_ip` - an ip/netmask which will be shared among
network hosts on eth0.

`floating_ip` - an optional ip/netmask which will be failed-over on
interface br-ex whenever `undercloud_floating_ip` fails over.

`neutron_gateway` - an optional ip/netmask which will be failed-over on
interface br-ex whenever `undercloud_floating_ip` fails over.

Additionally, add a sensu check which will send a critical alert whenever
a node changes from ucarp state BACKUP -> MASTER.