Call config_parse in base test setup

Call config_parse in base test setup

Some of the tests (e.g. NeutronDbPluginV2AsMixinTestCase) do not call
config_parse so if the database engine is not already been setup by another
test before the config object is reset on teardown, the database connection
will fail since the database config is then empty.

This patch adds a new setup_config method called during the base test case
setUp method which calls config_parse by default to load the default config.
Tests that couldn't use the default config were then modified to override the
setup_config method.

Some other unit tests were slightly adjusted to pass using the default config.

Closes-Bug: #1346673
Change-Id: I3724200fa932115c0a8c264640b1a9dbe431a1cc