OCP 4: do not try to delete DNS records if there are none

This commit fixes the error:

TASK [Delete all record from the zone] *****************************************
Tuesday 26 February 2019  01:06:36 -0500 (0:00:00.276)       0:43:40.451 ******
fatal: [localhost]: FAILED! => {"changed": true, "cmd": ["aws", "route53",
"change-resource-record-sets", "--hosted-zone-id=Z2Q4202QQ6KZW4",
"--change-batch=file:///...workdir/bd94_Z2Q4202QQ6KZW4_delete_records.json"],
"delta": "0:00:00.336782", "end": "2019-02-26 01:06:37.028327",
"msg": "non-zero return code",
"rc": 255, "start":
"2019-02-26 01:06:36.691545",
"stderr": "\nParameter validation failed:\nInvalid length for parameter
ChangeBatch.Changes, value: 0, valid range: 1-inf", "stderr_lines":
["", "Parameter validation failed:", "Invalid length for parameter
ChangeBatch.Changes, value: 0, valid range: 1-inf"], "stdout": "",
"stdout_lines": []}