Rename instance_type to flavor in nova.network tree

Renames 'instance_type' to 'flavor' in the nova.network tree along with
related tests.

Note that 'instance_type_id' is part of the Instance table in the data
model and is not touched as part of this change.

Part of blueprint flavor-instance-type-dedup

Change-Id: I7319315daa77b7334f6d5509cf87faa71148f8ef
Merge "Rename instance_type to flavor in nova.network tree"