Add attributes for swift_enable_snet & swift_store_region

- Add default["openstack"]["image"]["api"]["swift"]["enable_snet"] attribute
- Add default["openstack"]["image"]["api"]["swift"]["store_region"] attribute
- Set defaults such that the end result doesn't change, but attributes
  can be overridden.
- Add tests for these 2 attributes
- Add README.md entries for these 2 attributes

Change-Id: I8dd10a9731f996ebe6b064d2552f9cc5a1bf468d
Closes-Bug: #1274937
Merge "Add attributes for swift_enable_snet & swift_store_region"