Support snap installed LXD for nova-lxd/OpenStack

Track the enablement of a snapped install of LXD across nova-lxd, lxd and pylxd.

The nova-lxd changes need to be backported to bionic/rocky, and possibly bionic/queens, as a snap install of LXD is possible and supported/preferred on bionic.

nova-lxd master review: https://review.openstack.org/#/c/640317/ -- merged
nova-lxd stable/rocky: https://review.openstack.org/#/c/649667/ -- merged

nova-compute charm: https://review.openstack.org/#/c/640439 -- not yet merged

charm-specs: https://review.openstack.org/#/c/635484/ -- not yet merged

lxd charm: https://review.openstack.org/#/c/640429/ -- not yet merged

SRU bug for stable/rocky 18.0.1 tagged release for cosmic: https://bugs.launchpad.net/ubuntu/+source/nova-lxd/+bug/1825373

The nova-lxd backports (to at least rocky) should be landed prior to the lxd charm, as otherwise an upgrade (and then subsequent attempt at upgrading lxd to a snapped version) will cause a break on the associate nova-compute juju application.