zdtm: set cpu.cfs_period_us to 100 during sched_policy00 test

To run the test zdtm/static/sched_policy00
/sys/fs/cgroup/cpu/user.slice/cpu.cfs_period_us needs to be non zero.
This changes it to 100 during the test and resets it to the original
value once the test finishes.

travis-ci: success for zdtm: set cpu.cfs_period_us to 100 during sched_policy00 test
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>