pbx: update SIP config to help deal with NAT issues

SIP is pretty much terrible at dealing with NAT.  Asterisk has some
knobs that can be enabled to help deal with common issues.  Turn them
on.

In passing, remove the videosupport=yes line.  We don't have video
enabled for the conference application and we don't have any video
codecs allowed, so this option didn't do anything.

Change-Id: Ibc17ad3da9bbc110a8cb19daaea1655d0a208670
Merge "pbx: update SIP config to help deal with NAT issues"