Allow unit tests to be run independently

Allow unit tests to be run independently

Add various initializations and imports so that unit tests can be run
independently.

This change fixes the following test cases which could not be run
independently, that is running any individual unit test case by going
in to the py27 venv and running
"unit2 neutron.tests.unit.module.Class.test_case":

 neutron.tests.unit.plugins.ml2.drivers.arista.test_mechanism_arista.*
 neutron.tests.unit.plugins.ml2.drivers.cisco.apic.*
 neutron.tests.unit.plugins.ml2.test_rpc.RpcCallbacksTestCase.*
 neutron.tests.unit.services.l3_router.test_l3_apic.*
 neutron.tests.unit.agent.dhcp.test_agent.TestDhcpAgentEventHandler.*

(Note that these issues are not seen when running tox because the
initializations occur when all test modules are imported for test
discovery.)

Closes-bug: 1438463
Closes-bug: 1454640

Change-Id: I681caa66b51ce9a7bfbee5dfc43d534ba0d51947