Add Puppet 3 client compatibility under rack

Add a Rack middleware re-implementing the v1 and v2 API calls, as
per[1]. This should restore compatibility with Puppet 3 clients when
run under any rack-enabled server (including Passenger).

Closes: #832536

[1] https://github.com/puppetlabs/puppetserver/blob/puppet-server-2.6.0/src/clj/puppetlabs/services/legacy_routes/legacy_routes_core.clj