Description of problem:
Applying patches to an Openstack platform can be long to proceed.
The only supported way is to apply all the steps of the "overcloud update" documentation [1]

A customer who runs the latest zstream and wants to apply a discrete RHEL patch (a pacemaker bugfix, BZ 1972369, for example) has to run the entire overcloud update procedure.
Only the controllers will receive updates, but every overcloud node will be targeted by heat and ansible.
The customer says they will spend a day updating their platform, and they plan to grow to ~150 compute nodes, and the time to apply patches if needed will grow.

Version-Release number of selected component (if applicable):
RHOSP 16.1+



Expected results:
Provide a way to update only roles (controllers, compute, storage, ...)


Additional info:

[1] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html-single/keeping_red_hat_openstack_platform_updated/index#assembly-updating_the_overcloud