Core/RBAC: Allow setting different default security level roles to diffe...

...rent realms

Currently if you have more than a single realm using the same auth database you're limited when it comes to assigning default permissions that differ from one realm to another. 

You can assign them using rbac_account_permission but that means you must assign the given role/permission to every account individually and that proves to be very bothersome to manage in the long term.
how does this handle the case of permissions set for current realm and -1 in database ? Which is the effective value used at runtime ?