Allow swift_vars to not be specified for swift_hosts

* Default swift_vars to an empty list when undefined
* Allows default to work as expected.
* Avoids failures based on swift_vars not existing (even though values were already defaulted)
* Using default statements fail if the "parent" var doesn't exist.

Change-Id: I27eb876a1a0110bfe5cb44150c7c2a96c4313f60
Closes-Bug: #1402594
Merge "Allow swift_vars to not be specified for swift_hosts"