Add -K option to specify an external juju controller

How to use: Bootstrap an external controller (p.e. manual), and put -K <controller> name in the installer
Requirements:
  - Port 8443 accessible for the LXD Cluster
  - Port 17070 accesible for the Juju controller
Note: You can use -l and -L options to specify an external LXD cluster too.

Change-Id: I93db94f3c57f32f2112c0394e2685577f7a1eee1
Signed-off-by: David Garcia <david.garcia@canonical.com>