Add auth_policy option to jabber.conf for auto user registration.

The option is global and currently the acceptable values as noted in the sample
config are accept or deny.

(closes issue #15228)
Reported by: lp0


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@235342 65c4cc65-6c06-0410-ace0-fbb531ad65f3