Upgrade Pre-requisite and dealing with etcdv3/kdd backend error

[moshiurH]

If the user attempts to upgrade MKE 3 with the typical etcdv3 backend (most common backend for MKE 3), they will run into the following error:

$ mkectl upgrade --hosts-path hosts.yaml --mke3-admin-username admin --mke3-admin-password orcaorcaorca -l debug --config-out new-mke4.yaml --external-address "147.75.202.233"
...
Error: unable to generate upgrade config: unsupported configuration for mke4 upgrade: mke3 cluster is using etcdv3 and not kdd backend for calico

To circumvent this issue, set the following flag in the toml flag as true:
calico_kdd = false
to
calico_kdd = true

$ AUTHTOKEN=$(curl --silent --insecure --data '{"username":"'$MKE_USERNAME'","password":"'$MKE_PASSWORD'"}' https://$MKE_HOST/auth/login | jq --raw-output .auth_token)

$ curl --silent --insecure -X PUT -H "accept: application/toml" -H "Authorization: Bearer $AUTHTOKEN" --upload-file 'mke-config.toml' https://$MKE_HOST/api/ucp/config-toml
{"message":"Calico datastore migration from etcd to kdd successful"}