Note that the APIs that would change are admin-only by default. So in
this case nova is configured with a service user to check if the
requested project_id exists on behalf of the (admin) user making the
compute API request to add/remove flavor access (or update quota values
for a project). The service user does not have enough permissions in
keystone to check if the project exists. Option 1 is give that service
user more authority. Option 2 is basically re-raise that error to the
compute (admin) user to let them know they basically need to fix their
deployment (option 1 again).
A combo of both solutions where we raise the error to the user and amend our docs to help them fix it seems good to me.
I don't think a microversion is necessary for this
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