lxc: add possibility to define init uid/gid

lxc: add possibility to define init uid/gid

Users may want to run the init command of a container as a special
user / group. This is achieved by adding <inituser> and <initgroup>
elements. Note that the user can either provide a name or an ID to
specify the user / group to be used.

This commit also fixes a side effect of being able to run the command
as a non-root user: the user needs rights on the tty to allow shell
job control.

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>