Disable DNS in chutney by default, and add an option to enable it

Due to legacy/trac#21900 (moved), we need to explicitly specify ServerDNSResolvConfFile /dev/null for chutney to work on macOS when the network is unavailable.

We should also set ServerDNSDetectHijacking 0.

This should be the default, because:

• users who run chutney might not want it using DNS in a detectable pattern, and
• it makes chutney more reliable, because it no longer depends on a working DNS.

Some users will want chutney to be able to use hostnames, so we should add a tools/test-network.sh option and environmental variable to re-enable the default ServerDNSResolvConfFile setting (or, even better, use a custom ServerDNSResolvConfFile).

There seems to be no reason to turn on ServerDNSDetectHijacking: some users might be using chutney with internal DNS names.