define list container api (v2)

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.
If a char ** is passed in, then an array of container names
is returned, after which the caller must free all the names
and the name array, as showsn in the testcase.

Changelog:
	Check for the container config file before trying to
	create an lxc_container *, to save some work. [ per
	stgraber comments]
	Add names ** argument to return only container names.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>