ipadnsrecord: fix record modification behavior.

When modifying a record, depending on how the playbook tasks were
arranged, it was possible to end with more records than expected.

This behavior was fixed by modifying the way records are searched
when a modification is requested. This change also allows less calls
find_dnsrecord.

Tests were modified to reflect the changes, and a new test playbook
was added:

    tests/dnsrecord/test_dnsrecord_modify_record.yml