Fix 'asert' typo in unit test

Fix 'asert' typo in unit test

The dellscapi unit tests contain two occurences of an
apparent typo, 'mock_get_volume_list.asert_called_once_with'.
Because mock will magically create an 'asert_called_once_with'
method, this typo masks a problem with the test_manage_unmanage
test where the wrong set of arguments is expected.

This commit substitutes 'assert' for 'asert' in these occurences
and fixes up the test_manage_unmanage test.

Change-Id: I0f1b370ef61b28a46ab48d40595c79ec13e359f8