controllers/cpuacct: add cpuacct tests to default and somecleanup

* Change the entry like:
	cpuacct01 run_cpuacct_test.sh 1
	cpuacct02 run_cpuacct_test.sh 2

* Add cpuacct tests to scenario_groups/default

* Add umount_cpuacct_mounted function for umount cpuacct mounted

* Make use of tst_require_root, exists and tst_resm

* Some cleanup

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>