Add pyyaml to proposal slave

The script tools/normalize_projects_yaml.py in project-config is run as
periodic job on the proposal slave. It needs the Python yaml module, so
install it.

Change-Id: I113dcfa13da34b3981de8910e6d07d69c5f95dfc

modules/openstack_project/manifests/proposal_slave.pp

@@ -29,7 +29,7 @@ class openstack_project::proposal_slave (
   }
 
   package { ['Babel', 'pyopenssl', 'ndg-httpsclient', 'pyasn1',
-             'requestsexceptions']:
+             'pyyaml', 'requestsexceptions']:
     ensure   => latest,
     provider => pip,
     require  => Class['pip'],