Bug 1357562

It would be super nice if copr builders were less bound by copr backend, by
that I mean we should have a way to say (on builder):

  $ copr-build --copr USER/ID --DISTGIT-HASH .... [--copr-instance=fedora]

This command would be called by backend (through ssh/ansible over ssh) 
-> and thus there would be no need to play with constructing the ugly
'mockbuild' command on backend.  The backend code would be much simpler and
easy to maintain.

Some reasons to do so:
* there are people which would like to have project-dedicated builders (see the
bug 1334701), this should allow them easily to provide buidler 'IP' (there is
only need to have installed copr-builder, one config because builder needs to
know where backend/fronend/dist-git reside, and installed copr-builder package).

* lower the work done by dist-git --> we should be able to generate SRPMs on
builder so other machines should be less overloaded.

* build would be more reproducible, it would be very easy to run that
'copr-build' command on local box

* easier implementation of live logs (i would like to propose the idea very
soon)

* I'll provide more reasons probably :) I know there were some

I would very appreciate not closing this bug as WONTFIX or NOTABUG if we
don't have enough manpower to do this -- unless there are really good reasons
to not work on this.  Otherwise, I'm probably able to help here and hack on
making this done.

+++ This bug was initially created as a clone of Bug #1334701 +++