define list container api

Two new commands are defined: list_defined_containers() and
list_active_containers().  Both take an lxcpath (NULL means
use the default lxcpath) and return the number of containers
found.  If a lxc_container ** is passed in, then an array of
lxc_container's is returned, one for each container found.
The caller must then lxc_container_put() each container and
free the array, as shown in the new list testcase.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>