Move migration dir determination to common module

Also use it instead of __file__ magic in nailgun module.

Change-Id: Ie60f817158c1b70d433f4ee2f4218f98f1982dbe

tuning_box/__init__.py

@@ -13,7 +13,12 @@
 # under the License.
 
 import pbr.version
+import pkg_resources
 
 
 __version__ = pbr.version.VersionInfo(
     'tuning_box').version_string()
+
+
+def get_migrations_dir():
+    return pkg_resources.resource_filename('tuning_box', 'migrations')

tuning_box/nailgun.py

@@ -13,7 +13,6 @@
 from __future__ import absolute_import
 
 import itertools
-import os
 import threading
 
 from nailgun import db as nailgun_db
@@ -84,4 +83,4 @@ class Extension(extensions.BaseExtension):
 
     @classmethod
     def alembic_migrations_path(cls):
-        return os.path.join(os.path.dirname(__file__), 'migrations')
+        return tuning_box.get_migrations_dir()

tuning_box/tests/test_db.py

@@ -22,11 +22,11 @@ from alembic import script as alembic_script
 import flask
 from oslo_db.sqlalchemy import test_base
 from oslo_db.sqlalchemy import test_migrations
-import pkg_resources
 import sqlalchemy as sa
 import testscenarios
 from werkzeug import exceptions
 
+import tuning_box
 from tuning_box import db
 from tuning_box.tests import base
 
@@ -152,13 +152,11 @@ class _RealDBTest(testscenarios.WithScenarios,
         ('postgres', {'FIXTURE': test_base.PostgreSQLOpportunisticFixture}),
     ]
 
-    def get_migrations_dir(self):
-        return pkg_resources.resource_filename('tuning_box', 'migrations')
-
     def get_alembic_config(self, engine):
         config = alembic_config.Config()
         config.set_main_option('sqlalchemy.url', str(engine.url))
-        config.set_main_option('script_location', self.get_migrations_dir())
+        config.set_main_option(
+            'script_location', tuning_box.get_migrations_dir())
         config.set_main_option('version_table', 'alembic_version')
         return config