Add ability to specify alternate LXC config template file

Either:
  * to the server as a whole with the -T option with docker -d
  * on a specific run with the -T option to docker run
  * during container build with DockerLxcTemplate in the
    directory along side Dockerfile

Change-Id: I6cc0e342ecb81a1ff8a7448957d4b9b2668f2e04