Require python-novaclient>=2.15.0

Havana Cinder requires new novaclient functionality for
qemu-assisted-snapshots (create/delete_volume_snapshot) and
volume migration (update_server_volume).

Change-Id: I2368d8c2e2cd85aa914d81fbbd7c7146a34e5d4b
Merge "Require python-novaclient>=2.15.0"