Fix: Duplicate Discovery

The output of `knife cluster show p1` was highlighting an
error with the broker discovery process: when referencing
more than one cluster at a time, the ec2 machine/ebs_volumes
provider would load! more than once resulting in a false
discovery of 'duplicate' machines/volumes'.

This patch only load!s once, while still checking servers and
ebs_volumes for respective duplicates.

Squashed commit of the following:

commit fc8b451645355ac48c1da9ebeab535fca57a6b34
Author: Travis Dempsey <travis@infochimps.com>
Date:   Tue Apr 1 14:39:10 2014 -0500

    only load! servers and ebs_volumes once

    Change-Id: Idd53b947e3068b8fb0a6dd9b76eb0a6ab72cd1de

Change-Id: I971d292167ba413a1ca7003d5a64bc252f30dc9e