fixes issue #4956
previous code only worked for the first 10 domains on the account (as Linode
API returned a paginated response, with only 10 records).
This change makes an exact search query for each subdomain, completely removing any
need for walking through paginated responses. What makes it work for large
accounts with any number of domains.
reduce TTL for the TXT record from 3600 to 300 to have an easier way to replicate changes for the dns-verification in case multiple submissions for a specific record/domain are done within an hour.