Fix shared DNS recordset searching

[outcatcher]

Summary of the Pull Request

Add checking for exact equality of found DNS recordset name

Fix #847

PR Checklist

• Refers to: Can't create opentelekomcloud_dns_recordset_v2 for domains with subdomains #847
• Tests added/passed.

Acceptance Steps Performed

=== RUN   TestAccDNSV2RecordSet_basic
--- PASS: TestAccDNSV2RecordSet_basic (57.82s)
=== RUN   TestAccDNSV2RecordSet_childFirst
--- PASS: TestAccDNSV2RecordSet_childFirst (52.82s)
=== RUN   TestAccDNSV2RecordSet_readTTL
--- PASS: TestAccDNSV2RecordSet_readTTL (34.85s)
=== RUN   TestAccDNSV2RecordSet_timeout
--- PASS: TestAccDNSV2RecordSet_timeout (34.29s)
=== RUN   TestAccDNSV2RecordSet_shared
--- PASS: TestAccDNSV2RecordSet_shared (54.60s)
PASS

Process finished with exit code 0

[otc-zuul]

Build succeeded.

• golang-make-test : SUCCESS in 4m 46s
• golang-make-vet : SUCCESS in 2m 59s
• goreleaser-build : FAILURE in 7m 21s (non-voting)