Changes the way keyrotation is enabled by default

Key rotation was enabled by default by setting the global encryption method to IAX_ENCRYPT_KEYROTATE.  the problem with this is that if encryption is not enabled, and the encryption method is set to anything except 0, the peer appears to have encryption enabled when issuing a "iax2 show peers".  Rather than have the key rotation bit always set by default, it is now only set when an encryption method is enabled. 

(closes issue #14523)
Reported by: mvanbaak


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@178030 65c4cc65-6c06-0410-ace0-fbb531ad65f3