Make Rackspace Cloud DNS TTL constraints match API

The Rackspace Cloud DNS TTLs have a minimum constraint of 301, but,
according to the API docs, the minimum is "300 seconds or more".

Change-Id: I924889f859de4226d5e7517576e40d1fdbba3064
Closes-Bug: #1380755
Merge "Make Rackspace Cloud DNS TTL constraints match API"