Adds support for multihost nova-network

- Modifies Nova plugin to accept multiple hosts
in CONFIG_NOVA_NETWORK_HOSTS parameter.

- Fixes validate_multi_ip

Change-Id: I16bb63040b5825d4e8f7ed9a95d0ce46680e4074
Fixes: rhbz#1001916
Merge "Adds support for multihost nova-network"