NEXUS-5790: Added proper constraints to info and maven2 describe

As those should operate only on existing files, and maven2
only on existing file that lies on maven2 repo layout path.
NEXUS-5790: Info client follows redirects

But only one redirection is followed.
This is needed as new content resource does not
implement info and maven2 describes, but rather
redirects to legacy restlet1x resources.

This work nicely in browsers and UI, but ITs were
not following redirections, making them fail.