Add multiple-instance support to ckanbuild

[seanh]

Probably use ansible to do this. To create an instance, create a dir at /etc/ckan/MYSITE, and put MYSITE.wsgi, MYSITE.ini and who.ini files in it. Also put a MYSITE file in /etc/apache2/sites-available. See the example files already present in ckanbuild. Booting a new site should be a single command.

May not handle the postgres/solr/elastic-search side of things yet, could just require the user to set these up herself first and then pass them as args to the create-instance command.