Fix beaker test with package_provider error.

This commit[1] in puppet-stdlib adds the package_provider.  Since
puppetlabs-stdlib 4.11 in pulled out of the repo[2], it has this patch.
The declaration of the $package_provider variable is no longer possible.
This patch removes it and uses the fact.

Closes-Bug: 1568983

[1] e1a9bf5063
[2] http://logs.openstack.org/90/304190/1/check/gate-puppet-openstacklib-puppet-beaker-rspec-centos-7/9a2160a/console.html#_2016-04-11_15_55_23_678

Change-Id: I61edd49393694f2627cbe07064013dd75eef11a9

spec/acceptance/rabbitmq_spec.rb

@@ -13,15 +13,13 @@ describe 'openstacklib class' do
           release     => 'kilo',
           manage_epel => false,
         }
-        $package_provider = 'yum'
       } else {
         include ::apt
-        $package_provider = 'apt'
       }
 
       class { '::rabbitmq':
         delete_guest_user => true,
-        package_provider  => $package_provider
+        package_provider  => $::package_provider
       }
 
       # openstacklib resources