Avoid errors in log when neutron tests are skipped

The BaseNetworkTest class setUpClass() method will initialize
various class attributes which the tearDownClass() method
expects to exist. Unfortunately when Neutron is not
available, the entire of the setUpClass() method is skipped,
but tearDownClass() still runs. This causes the logs to get
polluted with errors about missing attributes during
tearDownClass(). eg

ERROR tempest.test [-] tearDownClass failed: type object
 'DHCPAgentSchedulersTestJSON' has no attribute 'ipsecpolicies'
TRACE tempest.test     for ipsecpolicy in cls.ipsecpolicies:

Make the tearDownClass() code be conditional on neutron
being enabled too.

Change-Id: I042ca5c47e5c1e8d1042754fd7f36036a40b2f03
Merge "Avoid errors in log when neutron tests are skipped"