Sync requirements with global requirements

* bump alembic to >=0.6.0
* bump python-novaclient to >=2.15.0
* remove version pin for six
* bump pyflakes to >=0.7.2,<0.7.4

Change-Id: Ib3e905d9dcf6c5e967389b1d14ac34564ef0293c
Merge "Sync requirements with global requirements"