Wrapper for updating Neutron resources

Currently users cannot describe actions for updating Neutron resources in their policy rules via the 'execute' operator.

A wrapper enabling updating of Neutron resources with arbitrary resource attributes is herein proposed. The wrapper would take a minimum of 1 resource attribute (key-value) to be updated (otherwise would not be updating anything at all):

 - resource name (e.g. 'port', 'network', 'subnet')
 - resource ID
 - key1-value1, key2-value2, ...

 Example:

 execute[neutronv2:update("port",
                                                  "25443f00-1ef4-47fd-b1fa-f494126e196e",
                                                  "admin_state_up", "False")]

Implementation-wise, it would be similar to the existing wrapper for servers.set_meta() in the Nova driver.

Refs:
- https://github.com/openstack/congress/blob/master/congress/datasources/nova_driver.py#L270
- https://github.com/openstack/python-neutronclient/blob/master/neutronclient/v2_0/client.py