Remove direct mirror_slave depends on python-yaml.

* modules/openstack_project/manifests/mirror_slave.pp: Remove the
package declaration for python-yaml since the jeepyb module, which
is already included, depends on it in a more platform-independent
manner anyway.

Change-Id: Iacdf1b0073089f193d8eff34ebcf50e6aa59e669
Reviewed-on: https://review.openstack.org/28655
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins