Suppress outputs in test_cli20_nsx_networkgateway

The output is from argparse and we need to capture stderr in
setUp(). oslotest.base.BaseTestCase is used as the base class
of test_cli20 to setup stdout/stderr capture.

Closes-Bug: #1296151
Change-Id: I86c50efcf451242c2c1ec13ac5def19d82c1e172
Merge "Suppress outputs in test_cli20_nsx_networkgateway"