ast_tls_cert: don't recreate generated files

Don't regenrate cat.cfg, ca.crt and ca.key if they were already created
on a previous run.

(closes issue ASTERISK-21932)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393383 65c4cc65-6c06-0410-ace0-fbb531ad65f3