SIP register not using peer's outbound proxy

If callbackextension is defined for a peer it successfully causes
a registration to occur, but the registration ignores the
outboundproxy settings for the peer.  This patch allows the
peer to be passed to obproxy_get() in transmit_register().

(closes issue #14344)
Reported by: Nick_Lewis
Patches:
      callbackextension_peer_trunk.diff uploaded by dvossel (license 671)
Tested by: dvossel

Review: https://reviewboard.asterisk.org/r/294/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@205985 65c4cc65-6c06-0410-ace0-fbb531ad65f3