provider/lxd: clean up client certificates

[axw]

We now record the fingerprint of the client
certificate in the instance metadata of
controller machines. When destroying the
machine, we check the metadata for a
certificate fingerprint, and then remove
the certificate with that fingerprint from
the server.

Because we did not previously record the
certificate fingerprint, this changes only
enables us to remove certificates for newly
created controllers.

Fixes https://bugs.launchpad.net/juju/+bug/1616346

QA

1. juju bootstrap lxd && juju enable-ha -m controller
2. juju destroy-controller -y lxd, ensure all certs removed
3. bootstrap two lxd controllers on the same machine
4. destroy one of them, and ensure only its certificates
   are removed

[reedobrien]

LGTM. I'd want someone with more (read any) lxdclient knowledge to also look.

QA checks out.

[reedobrien]

All unit tests pass

[axw]

$$merge$$

[jujubot]

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