(PUP-3644) Copy v1 to v3

This commit simply makes a copy of the v1 http handler code
to a new file called v3.rb; no code changes yet other than
changing the class name from `V1` to `V3`.  Doing this as
an isolated commit so that subsequent changes to the v3.rb
code will have a more meaningful diff in the VCS history.

It also copies the spec tests for v1 over to v3.
(PUP-3644) Move legacy routes to /v3 url namespace

This commit removes the "v1" HTTP API, which mounted
all of the various Puppet endpoints at "/" on the webserver.

In its place we now have the "v3" API, which includes
the versioned "/v3" prefix for all of the indirected URLs.