[Fleet] Policy monitor should work with policy id + namespace

[nchaulet]

Description

In the effort to support spaces, retrieving a policy during enrollment or checkin should be done with the policy_id and the namespaces, as we do not enforce unique id for different namespaces.

Currently we retrieve the policy only based on policy_id this could lead to retrieve the wrong policy if duplicate policies exists in different namespaces.

[nchaulet]

I think we may be able to close that one as moving to multiple policy allow to have unique id across spaces