common: Test settings improvements

- Avoid settings overrides from conf files inside SYNNEFO_SETTINGS_DIR
  Tests are expected to succeed with exact setting values defined either
  in tests modules/cases or in the default synnefo setting modules. To
  ensure this we temporarily set SYNNEFO_SETTINGS_DIR environmental variable
  to a non existing directory.
- Use DEBUG=False by default. We want our code to be tested in an
  settings environment as close to what we would set in a production
  deployment.
- Set initial astakos IM_MODULES setting to be able to test third party
  providers. This will be removed once we manage to dynamically run
  different tests with different IM_MODULES combinations

snf-common/synnefo/settings/test.py

+import os
+os.environ['SYNNEFO_SETTINGS_DIR'] = '/etc/synnefo-test-settings'
+
 from synnefo.settings import *
 
-DEBUG = True
+DEBUG = False
 TEST = True
 
 DATABASES = {
@@ -12,3 +15,7 @@ DATABASES = {
 
 LOGGING_SETUP['handlers']['console']['level'] = 'WARNING'
 LOGIN_URL = 'http://host:port/'
+
+SOUTH_TESTS_MIGRATE = bool(os.environ.get('SOUTH_TESTS_MIGRATE', True))
+
+ASTAKOS_IM_MODULES = ['local', 'shibboleth']