lxc-ubuntu.in: fix up the logic adding group for bound users

lxc-ubuntu.in: fix up the logic adding group for bound users

 1. 'getent group $user' assumes user's group is named $user.
 2. if 'getent group' returns error, just ignore the group in container
 3. (misc) while it happens to all work out fine anyway, don't do
    getent passwd $bindhome if $bindhome isn't defined.  (it will
    successfully return all password entries)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>