Safely remove python-requests on workers

* modules/openstack_project/manifests/thick_slave.pp: Effectively
reimplement package removal in Puppet for Red Hat based platforms to
work around https://bugzilla.redhat.com/show_bug.cgi?id=973375 when
python-requests is installed. This was causing image updates to fail
on CentOS machines where the provider preinstalled the package as a
dependency of cloud-init, resulting in an error message "Rpmdb
checksum is invalid: pkg checksums: python-requests...".

Change-Id: I5933f6cbbe8f0258bb443b7ce9c511c7c1072db2
Merge "Safely remove python-requests on workers"