Allow deploying sites via Aegir (not just drush site-install)

The current build.xml provides a "site-install" target which uses "drush site-install" to install the site.

This pull request converts "site-install" to "deploy" and allows you to choose in the build.properties which "deploy.type" you would like - either: drush or aegir. (Note: the "site-install" target is retained as an alias to deploy.)

I could envision even deploying to Pantheon or something else interesting in the future! :-)