Set -discover=0 in regtest framework

The regtest framework is local, so don't try to discover our external IP.  This should keep those writing regtests from spending time debugging to #4502.

I suspect this also caused the pulltester hang mentioned in #4485.

3rd party IP providers seem to be interpreting node stop/start activity, common in regtests, to be DoS, making shutdown problems more common that one might otherwise expect.