REST API password change improvement

To change a user's password via the REST API, /settings/rbac/users/local/<user_id>  https://docs.couchbase.com/server/current/rest-api/rbac.html#create-a-local-user-and-assign-roles 

We require that a user's current roles and specific alignment to buckets/scopes/collections is specified each time, which means these have to be retrieved then sent back to the API along with a new password. 

This is error prone and could cause issues, it would be preferable to be able to provide just a new password for a user via REST API without providing the additional user metadata each time. 

This is a minor change that doesn't require any compatibility changes, so it could fit into a small release like 7.1.1