Clean class variable in compute manager test case

Currently the pollster stub in compute manager test case keeps
information in class variable and does not clean it.
When multiple test cases want to check such information, it will fail
because class variable is not cleaned among different test cases
execution.

Add tearDown method to clean it up.

Change-Id: I72f70da7018a9b44c32b7c41061b6bd21ee3f103
Signed-off-by: Yunhong, Jiang <yunhong.jiang@intel.com>