Further noninteractive enhancements

- Ensure that the msmtp-mta prompt is only displayed for
  interactive installs, and that the defaults are sane.
- Add msmtp-mta choice to the summary
- Ensure the "continue" prompt is suppressed for all unattended installs
  not just cluster unattended
Remove legacy cluster network-fixup code

Now that all stage2 installs require a working network at boot, there
is no need to "fixup" the ifupdown config that netcfg generates.

The installer really shouldn't muck with anything on disk.  Networking
stuff specific to PXE installs should be handled in the postinstall
script, if we ever need to do this in the future.