1. fix inconsistent use of '--auth-key' (not --auth_key) which broke their
   usage
2. add --debug option to lxc-ubuntu (which does set -x to show what broke)
   (idea from Idea from lifeless and benji)
3. fix incorrect assumption about group with -b option.  User's default group
   may not be the same as username.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>