Currently we are using empty string as storage class for postgre and
prometheus pods when the user does not provide any value. However any
empty value as storage class has a special meaning in Kubernetes:
```
If storageClassName is set to an empty string ('') in the PVC, no
storage class will be used (i.e.; dynamic provisioning is disabled for
this PVC)
Existing, “Available”, PVs (that do not have a specified storageClassName)
will be considered for binding to the PVC.
```
This commit will avoid setting the storage class in the PVC declaration,
so the default storage class will be used.