[JUJU-3472] Lxd cert validation bug

[tlm]

A change introduced to Juju introduced the concept of re-using the HTTP client for LXD connections to multiple different servers. The LXD client however when passed it's own http server modifies the transport for the http client. This causes the same http client to keep having it's transport modified for different connections.

The result is that https validation fails for some lxd remotes because the transport is configured for a different server.

Checklist

If an item is not applicable, use ~strikethrough~.

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

To test this properly you need to LXD clouds (not the same cloud added twice) to the controller.

1. Bootstrap a controller to your first LXD cloud
2. Add a model called one
3. Add a machine to the model one
4. Add a second lxd cloud and credentials
5. Add a new model called two using the cloud created in step 4
6. Deploy a new machine to the two model.
7. Kill the controller

If everything work you will receive no error messages about x509 certs. If it's failing the controller model will fail to be destroyed because the http client was setup for the two model and the x509 certs will fail.

Documentation changes

N/A

Bug reference

https://bugs.launchpad.net/juju/+bug/2003135

[hpidcock]

Nice. LGTM

[hmlanigan]

/build

[hmlanigan]

/merge

[tlm]

/merge

[tlm]

/merge

[tlm]

/merge