Fixed iax2 key rotation backwards compatibility

Turns key rotation back on by default.  Added bit into encryption IE to indicate whether or not key rotation is supported or not. If it is not supported then it is not enabled, which insures backwards compatibility.  This eliminates the need for the keyrotate option in iax.conf, so it has been removed. 

Review: http://reviewboard.digium.com/r/159/ 


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@175597 65c4cc65-6c06-0410-ace0-fbb531ad65f3