Add lxd-cloud and lxd-credential options in installer (-l and -L)

Add lxd-cloud and lxd-credential options in installer (-l and -L)

This commit allows the installer to use an external LXD cluster instead of a local one.
Added two options:
 -l: This points to a file containing the lxd-cloud information for juju
 -L: This points to a file containing the lxd-cloud credential information
This url contains more info about the format of those files: https://juju.is/docs/lxd-cloud-advanced

Change-Id: Ia243f48e3e815d9ab267db3085ac842ffc691721
Signed-off-by: David Garcia <david.garcia@canonical.com>