[MIG] base external dbsource

* Migration of base_external_dbsource to odoo 9.0

* Fixing test error

* Moving test from yaml to python

* Fixing pylint error in test class

* Placeholder added to connection string text zone

* improving test coverage

base_external_dbsource/tests/__init__.py

@@ -0,0 +1,3 @@
+# -*- encoding: utf-8 -*-
+
+from . import test_create_dbsource