Register core_plugin for all tests in test_plugin
Register core_plugin for all tests in test_plugin

1. What is the problem
In test_plugin module, we define a stub class FakePlugin to
bypass real database access for unit tests. In some previous
test cases, FakePlugin object is directly created and then
we test its function, but after some changes in Neutron, we
also need to register the class path of FakePlugin to an
configuration option called core_plugin, otherwise some of
test cases will fail with "core_plugin not configured" error.

2. What is the solution to the problem
Register core_plugin for each test cases.

3. What the features need to be implemented to the Tricircle
   to realize the solution
Register core_plugin in setUp function then every test cases
can use this option.

Change-Id: Ibc2d81585ec8e67c3f56f864c4cbc5d2f0a0efa9

stackforge/tricircle,stackforge/tricircle,openstack/tricircle,openstack/tricircle