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: http://svn.asterisk.org/svn/asterisk/trunk@393383 f38db490-d61c-443f-a65b-d21fe96a405b