res_pjsip_messaging: send message to a default outbound endpoint

In some cases messages need to be sent to a direct URI (sip:<ip address>). This
patch adds in that support by using a default outbound endpoint.  When sending
messages, if no endpoint can be found then the default one is used.

To facilitate this a new default_outbound_endpoint option was added to the
globals section for pjsip.conf.

Review: https://reviewboard.asterisk.org/r/2944/
........

Merged revisions 403680 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403687 65c4cc65-6c06-0410-ace0-fbb531ad65f3