Fix handling of cpumask for docker driver. Raw driver still reports all
cpus. Will fix it to read cpumask for cgroup seperately.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Change test data generator to create string cpumasks.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)