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
Warn on tiny subnet

Log a warning when a user creates a /32 IPv4 subnet since
it will be useless.

Closes-Bug: #1341040
Change-Id: I6a39919e7ce4a39ba67f1ca79e55708566c086fc
Merge "Suppress outputs in test_cli20_nsx_networkgateway"
Merge "Warn on tiny subnet"