Fix centos subvertpy.

Parameter 'ensure' is already set on Package[subvertpy] by #<Puppet::Resource::Type:0x7f4be80da800> at /root/puppet-manifests/modules/twisted/manifests/buildslave.pp:36; cannot redefine at /root/puppet-manifests/modules/twisted/manifests/buildslave.pp:58 on node twisted-bot-el6-i-c09d4316.cloud.fedoraproject.org