Define bridge/port fixtures for OVS/LinuxBridge/Veth backends

Define bridge/port fixtures for OVS/LinuxBridge/Veth backends

This change defines for OVS, LinuxBridge and veth[1] bridge and port
fixture classes in order to handle bridge and port setUp/cleanUp.

It allows to simplify BaseOVSLinuxTestCase[2] and remove
BaseBridgeTestCase[2].

[1] veth backend simulates a bridge with a veth
[2] in neutron.tests.functional.agent.linux.base

Change-Id: If34c9a8fb6fa584fb1e30173ec619d1aac9701f9