api: Add functions for iterating through the cgroup struct

api: Add functions for iterating through the cgroup struct

Add functions to iterate through the cgroup structure:
	cgroup_get_controller_count() - returns the number of controllers
		within the cgroup struct
	cgroup_get_controller_by_index() - returns the cgroup_controller
		pointer corresponding with the requested index
	cgroup_get_controller_name() - given a cgroup_controller pointer,
		get the controller's name

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>