migrations: Implement AdoptResources for the ec2 provider

[babbageclunk]

Description of change

The AdoptResources environ method will be used at the end of a model migration to update the controller tag for all of the resources used by the model, so that if the source controller is subsequently destroyed then these resources won't be cleaned up with it.

In the ec2 provider we tag volumes, instances and security groups with the controller UUID. We collect all of the resource IDs from these and call CreateTags in the ec2 API (which will overwrite tags, and leave tags in place if they aren't specified).

QA steps

None yet - this method isn't called from the migration master yet.

Documentation changes

N/A

Bug reference

This is part of the fix for https://bugs.launchpad.net/juju/+bug/1648063

[axw]

Nice and simple, LGTM.

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju