Update packaging layout in response to select plugin

New select(2)-based plugin allows to ship highly portable IO plugin
with the core library (although not as performant on big numbers of
sockets/events, which is fit most of needs of network client).

Therefore we can simplify packaging layout. So that the user can
install completely working solution  with the following command

    # with headers to compile libcouchbase-based clients
    sudo apt-get install libcouchbase-dev
    sudo yum install libcouchbase-devel

    # without headers if in case of just binary depency
    sudo apt-get install libcouchbase2
    sudo yum install libcouchbase2

This patch is making following changes:

* Remove libcouchbase2-backend virtual package
  There no need in this package, because plugin packages selected more
  explicitly. This change doesn't influence on upgdade.

* Make libcouchbase2 package virtual. It will be alias to
  libcouchbase2-core

* Remove libcouchbase2-all virtual package, this package will just
  dissapear and and doesn't influence on upgrade

Change-Id: I090cea1ba1f6596f0eb92b10f6a4a0e7232235a8
Reviewed-on: http://review.couchbase.org/27539
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>