Allow user to select different visibilities

A service may have one default visibility, but the user should
be allowed to override that.  For example listing tenants is
an administrative function by default, but is also available
over the public interface.  This will work:

    python examples/list.py openstack/identity/v2/tenant.py --os-api-visibility public

Change-Id: I4e40e55f9f7c369a9c3e900712a89fdbc743ffcb
Merge "Allow user to select different visibilities"