the user and host parameters to install-site.sh should be optional with --dev

[mhl]

If you specify both --dev and a user (pmo, say) for install-site.sh the results
are unexpected. e.g. if you're logged into a fresh vagrant box as vagrant and
run:

sudo ./install-site.sh --dev pombola pmo pombola.127.0.0.1.xip.io

... then DIRECTORY is set to /home/vagrant and all of /home/vagrant is
chowned to pmo.pmo here:

• https://github.com/mysociety/commonlib/blob/master/bin/install-site.sh#L553

There's no real use for the USER or HOST options when you're using --dev
so they should be optional.