Use maas devices api for address allocation

[voidspace]

Where address allocation is enabled, along with MAAS > 1.8, the new devices API will be used for container address allocation and deallocation. This allows MAAS to track that IP addresses for containers are associated with their host, and if the host goes away (for example with "destroy-environment --force" then the associated devices should be deleted and their IP addresses released. This prevents IP addresses leaking when "destroy-environment --force" is used.

The devices API requires a MAC address, and the device must be created before the address can be allocated which we need to do before the container is started (as the address is used in the container template to configure networking). This means we need to generate MAC addresses, and use them in the container templates (both lxc and kvm).

This PR includes generating MAC addresses on the API server and storing them with IP addresses in state, to be used for both IP address allocation and releasing.

[bz2]

This proposal was landed as it is actually identical to #2749 which was instead intended to be a merge the other way.