Add users db security rules on clustered interface

[mikewallace1979]

Apply authentication_db security rules when authentication_db is on the clustered interface (5984).

COUCHDB-2452

[kxepal]

LGFM, though long chains of orelse leaves some strange feeling about, but no idea how to make them better.

[mikewallace1979]

though long chains of orelse leaves some strange feeling about

I did cringe a little when adding the second orelse but also couldn't come up with an alternative (other than an even-more-unwieldy nested case statement).

[davisp]

I haven't done a thorough review yet, but this seems related to PR #41