unittests: setup FLAGS.state_path properly: bug 938637

unittests: setup FLAGS.state_path properly: bug 938637

This patch fixes bug 938637
Quantum unittests doesn't pass with plugin (openvswitch, linuxbridge) in venv
as follows.
This is because config file under <quantum src>/etc/ can't be find due to
FLAGS.state_path points to <python-quantumclient src> which can be different
from <quantum src>.
Set FLAGS.state_path to <quantum src> when quantum unit tests.

 $ PLUGIN_DIR=quantum/plugins/openvswitch ./run_tests.sh -V
<snip>
ActionExtensionTest
    test_extended_action_for_adding_extra_data (quantum.tests.unit.test_extensions.ActionExtensionTest)ERROR
<snip>
======================================================================
ERROR: test_extended_action_for_adding_extra_data (quantum.tests.unit.test_extensions.ActionExtensionTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/quantum/tests/unit/test_extensions.py", line 212, in setUp
    self.extension_app = setup_extensions_test_app()
  File "/quantum-src/quantum/tests/unit/test_extensions.py", line 474, in setup_extensions_test_app
    return TestApp(setup_extensions_middleware(extension_manager))
  File "/quantum-src/quantum/tests/unit/test_extensions.py", line 469, in setup_extensions_middleware
    conf, app = config.load_paste_app('extensions_test_app', options, None)
  File "/quantum-src/.venv/src/python-quantumclient/quantum/common/config.py", line 316, in load_paste_app
    conf_file, conf = load_paste_config(app_name, options, args)
  File "/quantum-src/.venv/src/python-quantumclient/quantum/common/config.py", line 286, in load_paste_config
    "Cannot load application %s" % app_name)
RuntimeError: Unable to locate any configuration file. Cannot load application extensions_test_app
-------------------- >> begin captured logging << --------------------
quantum.extensions.extensions: INFO: Initializing extension manager.
quantum.extensions.extensions: INFO: Loading extension file: foxinsocks.py
quantum.extensions.extensions: DEBUG: Ext name: Fox In Socks
quantum.extensions.extensions: DEBUG: Ext alias: FOXNSOX
quantum.extensions.extensions: DEBUG: Ext description: The Fox In Socks Extension
quantum.extensions.extensions: DEBUG: Ext namespace: http://www.fox.in.socks/api/ext/pie/v1.0
quantum.extensions.extensions: DEBUG: Ext updated: 2011-01-22T13:25:27-06:00
quantum.extensions.extensions: WARNING: Loaded extension: FOXNSOX
quantum.extensions.extensions: INFO: Loading extension file: __init__.py
quantum.extensions.extensions: INFO: Loading extension file: foxinsocks.pyc
quantum.extensions.extensions: INFO: Loading extension file: __init__.pyc
--------------------- >> end captured logging << ---------------------

Change-Id: I3b8e1414b22b1a526468488fe885ac39f2e6c420