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 <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

ostap/lxc,ostap/lxc