LXD image support (no box)

It would be very useful to have support for direct image use (much like the AWS Vagrant plugin) where a dummy box is specified and the image details are specified in the provider.

For example:

config.vm.box 'dummy'
config.vm.provider 'lxd' do |lxd, override|
  lxd.name = 'my-ubuntu-16'
  lxd.image = 'ubuntu-daily:xenial'

  # optionally create users and add private keys using #cloud-config in the profile if they're
  # not baked in to the image
  lxd.profiles = ['private-key-cloud-config']

  # [...]
  override.ssh.username = 'ubuntu'
  override.ssh.private_key_path = '/path/to/private.key'
end

This would make it much easier to leverage existing LXD image infrastructure without also needing to build/update/upload/download/manage Vagrant boxes separately.

These are the potential issues I can think of:

• Remotes are a client-only concept handled by lxc so the image details (server, secret, protocol, etc) may need to be specified explicitly for remote images
• Using #cloud-config to configure the remote user may not always yield a determinate uid/gid (required in lxd.vagrant_uid/lxd.vagrant_gid)