Agent resource provider config API calls should be idempotent.

There are some issues w.r.t. using the current agent resource provider config API calls:

1. UPDATE_RESOURCE_PROVIDER_CONFIG: If the caller fail to receive the HTTP response code, there is no way to retry the operation without triggering an RP restart.
2. REMOVE_RESOURCE_PROVIDER_CONFIG: If the caller fail to receive the HTTP response code, a retry will return a 404 Not Found. But due to MESOS-7697, there is no way for the caller to know if the 404 is due to a previous successful config removal or not.

To address these issues, we should make these calls idempotent, such that they return 200 OK when the caller retry. It would be nice if ADD_RESOURCE_PROVIDER_CONFIG is also idempotent for consistency.