add defaultNetstreamDriverCertFile option to the global() object

I didn't see an equivalent of $DefaultNetstreamDriverCertFile in the [global object](http://www.rsyslog.com/doc/global.html). This was the only missing piece for me to [configure TLS in rsyslog](http://www.rsyslog.com/doc/rsyslog_secure_tls.html) with the [new config format (RainerScript)](http://www.rsyslog.com/doc/rainerscript.html).

So I've added it. Again, don't trust me because I'm a newbie, so I just copy-pasted and changed what made sense to me. And I tested it on my machine and it works. Here's my config on the server side:

    module(  # TCP listener for TLS traffic
      load="imtcp"
      StreamDriver.AuthMode="anon" # don't try to authenticate clients
      StreamDriver.Mode="1" # run driver in TLS-only mode
    )
 
    # make gtls driver the default and show it the certificates
    global (
     defaultNetstreamDriver="gtls"
     defaultNetstreamDriverCAFile="/root/rsyslog-tls/ca.pem"
     defaultNetstreamDriverKeyFile="/root/rsyslog-tls/machine-key.pem"
     defaultNetstreamDriverCertFile="/root/rsyslog-tls/machine-cert.pem"
    )
 
    input( # start up TLS listener
      type="imtcp"
      port="10514"
    )
 
    action( # write everything to a file
      type="omfile"
      file="/var/log/messages"
    )

As always, I'll contribute the doc counterpart if/when the code commit turns out to be OK :)