Add multi-tenant SVM support

[cgrinds]

The goal is to restrict what information Harvest pulls for multi-tenancy.

What's been attempted:

• a) Point Harvest at the cluster IP but only provide SVM credentials
• b) Point Harvest at the SVM IP fails since Harvest still tries to send cluster ZAPIs

{"level":"warn","Poller":"netapp-ontap","time":"2021-07-26T17:54:32-07:00",
"message":"init collector-object (ZapiPerf:SystemNode): connection error => 
api request rejected => Unable to find API: cluster-identity-get"}

[cgrinds]

We should be able to make b work with some changes to Harvest. Perhaps something like this:

• Add a new svm_tenant key/value to harvest.yml

Pollers:
  infinity:
    datacenter: nane1
    svm_tenant: svm1
    exporters:
      - prometheus1

• Change the collectors to use svm tunneling when that keys is present

[chrishenzie]

Yes this design would work, so long as it allows us to use SVM credentials to fetch that SVM's specific metrics.

Would this require any changes to ONTAP?

[cgrinds]

should be achievable Harvest side, no ONTAP changes required