Allow snapcraft to use remote lxd instances.

[jat-canonical]

What needs to get done

Since snap builds only require a linux system container (LXD Container), could we look into making use of a remote LXD instance and relying on locally configured lxc configuration to reach out to the LXD instance to build snaps?

Why it needs to get done

I use a mac for my local development and run a remote lxd instance which can be reached using lxc to interact with the instance. Currently snapcraft looks for a local installation of lxd which is not possible on a mac. My current workaround is to to have dedicated linux vm for building snaps, have my local directory mounted in the vm and always login to the instance first to start building the snap. This might prove useful for any user not running linux as a daily driver host os but trying to develop snaps.