Fix format of sample-user.yml and change to generic creds

[kshithijiyer-px]

What this PR does / why we need it:
The current sample file when used fails with the below error:

Events:
  Type     Reason                Age                From                                                                               Message
  ----     ------                ----               ----                                                                               -------
  Normal   ExternalProvisioning  14s (x3 over 31s)  persistentvolume-controller                                                        waiting for a volume to be created, either by external provisioner "pxd.portworx.com" or manually created by system administrator
  Normal   Provisioning          2s (x6 over 31s)   pxd.portworx.com_px-csi-ext-67b5489d47-v9fj4_53ca019c-f1dc-4b4c-8db8-540cf1fb46b6  External provisioner is provisioning volume for claim "postgres/postgres-data"
  Warning  ProvisioningFailed    2s (x6 over 31s)   pxd.portworx.com_px-csi-ext-67b5489d47-v9fj4_53ca019c-f1dc-4b4c-8db8-540cf1fb46b6  failed to provision volume with StorageClass "postgres-sc-2": rpc error: code = PermissionDenied desc = signature is invalid

Correcting the format of the file and making it generic.

Which issue(s) this PR fixes (optional)
Closes #

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