Revert "Remove 'opam remote' 

This breaks quite a lot of stuff: all the Jenkins scripts, the xen.org instructions, and Mirage instructions too.  I'd strongly suggest keeping this alias and not bothering with `opam repository` (which I didn't realise existed, so all the docs I've written have been for `opam remote`).

`remote` also matches the equivalent `git` terminology, which has to be a good thing...