Set temporary version cap on oslo.db

This commit temporarily adds a cap on oslo.db because of the use of a
private method from oslo.db in the alembic env.py for subunit2sql
which was removed in the latest release. In the future we will switch
to using a public api from oslo.db for doing this. However, one does
not exist at this time. So we add a cap on the version of oslo.db to
ensure that a version including the private api is used.

Change-Id: I726a4afb9b4ed0d7283b42e8f5f45035fcf4fabf

requirements.txt

@@ -1,6 +1,6 @@
 alembic>=0.4.1
 oslo.config>=1.4.0.0a3
-oslo.db!=1.12.0
+oslo.db!=1.12.0,<2.0.0
 pbr>=1.0.0
 python-subunit>=0.0.18
 six>=1.5.2