ftests: Add cgget test with '-r' flag and two cgroups

ftests: Add cgget test with '-r' flag and two cgroups

Add a test that exercises the '-r' <name> <cgroup1> <cgroup2>
option in cgget.  This test verifies the following cgget
operation:

	$ cgget -r memory.limit_in_bytes 011cgget1 011cgget2

-----------------------------------------------------------------
Test Results:
        Run Date:                          Feb 09 21:24:10
        Passed:                                  1 test(s)
        Skipped:                                 0 test(s)
        Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
        Test                                    Time (sec)
        ---------------------------------------------------------
        setup                                        17.28
        011-cgget-r_flag_two_cgroups.py               1.47
        teardown                                      0.00
        ---------------------------------------------------------
        Total Run Time                               18.76

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>