introduce a prefix (wsgi_alias) into ::vhost

This introduces a prefix (analogous to ::conf, we call it $wsgi_alias)
into puppetboard::apache::vhost, which allows us to relocate puppetboard
under a different sub-URI.
This is useful in reverse-proxy scenarios where we may want to collect a
number of dashboards (or other useful applications) under a single
domain, each running under a different sub-URI.

It also indirectly fixes https://github.com/nedap/puppetboard/issues/74