Add support for repo based http proxy

[alwiesner]

This will not work behind a proxy.

  package { 'newrelic-repo-5-3.noarch':
    ensure   => present,
    source   => 'http://yum.newrelic.com/pub/newrelic/el5/x86_64/newrelic-repo-5-3.noarch.rpm',
    provider => rpm,
  }

It would be helpful to have an extra parameter to define an httpproxy:port that can be passed to the rpm command. Without that, systems that are behind a proxy and are precluded for adding 'global' http proxy exports will fail with this implementation.