Provide support for alternative site verification method

[dhivah]

The current provider supports only DNS_TXT record for site verification. Could this be changed to configurable field So as to support DNS_CNAME for site verification.

[dhivah]

@hectorj I need the verification method configurable because I am getting the following error trying to create TXT record and using CNAME verification resolves the issue.

Error creating DNS RecordSet: googleapi: Error 400: The resource record set 'entity.change.additions[0]' is invalid because the DNS name may have either one CNAME resource record set or resource record sets of other types, but not both., cnameResourceRecordSetConflict