Set alembic>=0.7.1

Alembic 0.7.1 contains some new functions like autogeneration of
foreign keys that are important and can reduce some code in neutron
and oslo.db

Change-Id: Ifc618a63ee0df6ac5c1b3a2b2f4176ebb9a1e080
Related-bug: #1398417
Merge "Set alembic>=0.7.1"