allow less strict cpu flag comparison

[norman shen]

Description
===========

Nova uses strict cpu flag comparison during live migration, this introduces some problems when
migrating with some cpu flags which do not affect actually migration. For example, `monitoring` flag
could be neglected safely.

So I think it might be reasonable to ignore some features provided from user input, whether static by configuration or dynamically from api input.

[Balazs Gibizer (balazs-gibizer)]

This sounds like a feature request. Could you please follow the spec process described in https://docs.openstack.org/nova/latest/contributor/blueprints.html ? If you need help with that then please joint to the #openstack-nova IRC channel on Freenode and ask for guidance there.

Marking this bug as whishlist to indicate that this is a feature request.