Add task to select machines by puppet class

This allows things like

    fab staging puppet_class:govuk_elasticsearch numbered:1
    es.safe_reboot

or

    fab staging puppet_class:govuk::safe_to_reboot::yes numbered:2 vm.reboot

though I think this last command needs more thought as it includes
mongos and elasticsearches which need a bit better handling.
Merge pull request #61 from alphagov/select_machine_by_puppet_class

Add task to select machines by puppet class