Refined schema for VPCZoneIdentifier

The schema now says it is a list of strings. Also upgraded the
description.  Also clarified the name of
test_invalid_vpc_zone_identifier.

Change-Id: I4b17dbd146965df3df3f4205dddafc0b65061294
Closes-Bug: #1294717
Merge "Refined schema for VPCZoneIdentifier"