Fix: Wrong assumption about possible CPUs

Fix: Wrong assumption about possible CPUs

cpuset is not necessary contiguous.  Thus, we need to parse
/sys/devices/system/cpu/posssible correctly.

Also, the `get_exposed_cpus_list' utility functions is required for
taskset-ting on the correct CPUs available to the test environment.

Change-Id: I062ce8d311ff0e8c4b757fe6f36387e3007cfa27
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>