LXD support uses remotes inconsistently when capturing images/launching instances

I use a non-"local" remote for my day-to-day LXD work. The `pycloudlib` LXD support sometimes specifies no remote (meaning my default remote is honoured) but sometimes explicitly specifies the "local" remote.

Concretely, when launching from a PPA I see that images are captured into my default remote, but are then attempted to be launched from the "local" remote:

```
INFO integration_testing:clouds.py:64 Launching instance with launch_kwargs:
image_id=local:welcomed-ape-snapshot
user_data=None
wait=False
ERROR
```