Changes

asterisk: Initial IMS registration

This patch is a step towards testing IMS.
So far only the code infrstrastructure to handle the 1st REGISTER
(outside IPSEC) is provided. It can already be seen how Asterisk
sends the second REGISTER over IPSEC, but there's no means to test it
yet in TTCN-3. This will be done in a follow-up patch, which may take
some work.

Change-Id: Idb3b19ccd82cad25948106b2c72aa424d7f79cd8