Use a private network instead of port forwarding

...also do some cleanup, including building tmux earlier so it's available
when I start commenting steps out (so we can do them by hand in the demo).
Install sendmail so gerrit can send email messages

e.g., to register new email addresses, etc.
config.vm.network belongs _outside_ config.vm.define
Merge branch 'master' into by-hand
Don't even call install-(postgres|gerrit).sh directly: do it all by hand

Because of that, download the gerrit .war file early.  We'll copy it to
the right place by hand after we create the gerrit2 user.  No need to
comment out the contents of install-gerrit.sh now that we don't call it
at all.