Rename instance_type to flavor in nova.cloudpipe

Renames 'instance_type' to 'flavor' in the nova.cloudpipe module.

The deprecated vpn_instance_type option is removed as part of
change I64548e40f so I leave it untouched here.

Part of blueprint flavor-instance-type-dedup

Change-Id: Id4fd757fabfb81ee81999e7978c924aabd0a712b
Merge "Rename instance_type to flavor in nova.cloudpipe"