Move _check_resize to nova_utils

Refactoring as part of blueprint native-nova-instance.

Change-Id: Ice9a4541316ad04ac04b24ac877e0fc3fbeae9c8
Use nova_utils for keypair validation

This removes unnecessary code.

Change-Id: Ide1d12a44213ffd8b7bd414eb00a4d12e3f6cc2a