Make ServiceInstance.cluster_names ordered
Make ServiceInstance.cluster_names ordered

This matches the behavior of .client_fqdns, and makes the contents of
service plenaries more stable.

Change-Id: I2c104d1c6834f21a91ae0881cc72ba4798c5349f
Reviewed-by: Dave Reeve <6a84ee3ae0690d4584b99406fb3076392171f50e@morganstanley.com>

quattor/aquilon,guillaume-philippon/aquilon,quattor/aquilon,guillaume-philippon/aquilon,guillaume-philippon/aquilon,quattor/aquilon