Handle Rackspace pagination of DNS records.

Rackspace paginates DNS records returned from their API. In list_records, query
Rackspace multiple times, if needed, to get the full list of records.
Move paginated DNS query code to iterate_records.
Remove unused variable.

Fix lint warning.
Merge branch 'fix-rackspace-list-records' of https://github.com/thanatos/libcloud into trunk