Add logic to ONTAP drivers to validate aggregate name

[ntap-rippy]

If the aggregate name is wrong, the underlying ZAPI will error with "entry doesn't exist".

Add validation logic around the aggregate.

Aug 24 23:00:14 gngqnc006e dockerd[44024]: time="2017-08-24T23:00:14+05:30" level=info msg="time="2017-08-24T17:30:14Z" level=debug msg="response Body:\n\n\n\n<results reason=\"entry doesn't exist\" status=\"failed\" errno=\"15661\"/>"" plugin=e5ddf0f114a7d690221a9d04290ca52a6d39fba13e2298b2f0d92cb27689c875