Add update_pkg_list_cmd to openstack_project.

Add a new update_pkg_list_cmd parameter to the openstack_project
module so we can control what each distribution runs to
update package lists. On Ubuntu we want to run 'apt-get update'
and on RHEL we want to run nothing since yum does this automatically.

Change-Id: Iab08930bac5f992711b05bcbfa23b7daab12a060
Reviewed-on: https://review.openstack.org/23571
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins