Fix resource cleanup in NetworkManager.allocate_fixed_ip

NetworkManager.allocate_fixed_ip isn't cleaning up the resources it
allocates in the event of a failure. This means that if, for example,
_setup_network_on_host fails, the instance will still have a fixed IP
allocated to it. Every attempt to spawn the instance will result in
another fixed ip being allocated.

Closes-Bug: #1328539

Change-Id: Idfa1f4015acb10dc14fcec448257bcfe6971fc99
Merge "Fix resource cleanup in NetworkManager.allocate_fixed_ip"