Added opportunistic DB test cases

These test cases are meant to be run opportunistically within the
general test suite, i.e. only when openstack_citest database is
available (which is always true for our CI infrastructure, but might not
be true for developers machines). This is similar to how we run
migrations tests in all OpenStack projects

Blueprint: tests-given-db-backend
Change-Id: Ic3706763c8fc56c0fcf4d1964ed1c5f17a662be6

tests/unit/db/sqlalchemy/base.py

@@ -13,13 +13,16 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import abc
 from functools import wraps
 import os
 
 import fixtures
 from oslo.config import cfg
+import six
 
 from openstack.common.db.sqlalchemy import session
+from openstack.common.db.sqlalchemy import test_migrations as tm
 from tests import utils as test_utils
 
 
@@ -90,3 +93,62 @@ def backend_specific(*dialects):
                 return f(self)
         return ins_wrap
     return wrap
+
+
+@six.add_metaclass(abc.ABCMeta)
+class OpportunisticFixture(DbFixture):
+    """Base fixture to use default CI databases.
+
+    The databases exist in OpenStack CI infrastructure. But for the
+    correct functioning in local environment the databases must be
+    created manually.
+    """
+
+    DRIVER = abc.abstractproperty(lambda: None)
+    DBNAME = PASSWORD = USERNAME = 'openstack_citest'
+
+    def _get_uri(self):
+        return tm._get_connect_string(backend=self.DRIVER,
+                                      user=self.USERNAME,
+                                      passwd=self.PASSWORD,
+                                      database=self.DBNAME)
+
+
+@six.add_metaclass(abc.ABCMeta)
+class OpportunisticTestCase(DbTestCase):
+    """Base test case to use default CI databases.
+
+    The subclasses of the test case are running only when openstack_citest
+    database is available otherwise a tests will be skipped.
+    """
+
+    FIXTURE = abc.abstractproperty(lambda: None)
+
+    def setUp(self):
+        credentials = (
+            self.FIXTURE.DRIVER,
+            self.FIXTURE.USERNAME,
+            self.FIXTURE.PASSWORD,
+            self.FIXTURE.DBNAME)
+
+        if self.FIXTURE.DRIVER and not tm._is_backend_avail(*credentials):
+            msg = '%s backend is not available.' % self.FIXTURE.DRIVER
+            return self.skip(msg)
+
+        super(OpportunisticTestCase, self).setUp()
+
+
+class MySQLOpportunisticFixture(OpportunisticFixture):
+    DRIVER = 'mysql'
+
+
+class PostgreSQLOpportunisticFixture(OpportunisticFixture):
+    DRIVER = 'postgresql'
+
+
+class MySQLOpportunisticTestCase(OpportunisticTestCase):
+    FIXTURE = MySQLOpportunisticFixture
+
+
+class PostgreSQLOpportunisticTestCase(OpportunisticTestCase):
+    FIXTURE = PostgreSQLOpportunisticFixture