Updated LXD plugin to be able to load images from LXD image repository

[gabrik]

Added a feature to LXD plugin, now it is able to load images from the LXD image repository.
Just use as image uri in the descriptor something like lxd://<image tag>

eg.

    "image": {
        "uri": "lxd://alpine/3.6",
        "checksum": "",
        "format": ""
    }

This implements #89

[gabrik]

LXD plugin is now able to use image from LXD image repository. Use lxd://<image tag> in the Image uri for use images from LXD image repository