provider/maas: better handling of devices claim-sticky-ip-address failures and absence of reserved IP address

[Dimiter Naydenov]

See related MAAS bug #1519527.

With this PR https://github.com/juju/juju/pull/3730/ and its forward ports, Juju registers all LXC/KVM machines as MAAS devices, calls claim-sticky-ip-address on the device, and deletes the corresponding device when the LXC/KVM machine is removed.

However, Juju can handle better the case where claim-sticky-ip-address fails (the error is swallowed up the stack in ProvisionerAPI.allocateAddress unless it's a NotSupportedError) and on success the response should be parsed to get the reserved IP address to return it back to the worker (from PrepareContainerInterfaceInfo) and log it. If there's no IP in the response and it's a success (like the MAAS regression described in the related bug), it should be treated as a failure.

This way we can surface up the provider issue to the user and improve the reliability.

[Dimiter Naydenov (dimitern)]

IMO this is High instead of Critical, as it's more of an improvement than a new regression on Juju's side.

[Dimiter Naydenov (dimitern)]

Fix for 1.25 proposed: https://github.com/juju/juju/pull/3866

[Dimiter Naydenov (dimitern)]

Fix for master: https://github.com/juju/juju/pull/3876