[Integreatly] Add param for system-storage PVC storageClassName in amp.yml template

What
Add new parameter in amp.yml template (1) to specify a storageClassName for the system-storage PVC.

The default value of the param should be left as blank so it will use the standard storage option by default unless stated otherwise.

Why
As part of the Integreatly initiative there is a requirement to install 3Scale on AWS hosted Openshift clusters. These clusters have been configured with EBS as the default storage option alongside EFS for RWX PV support.

We currently provision 3Scale using the standard amp.yml template (1) but have had to tweak it slightly to specify the EFS storageClassName for the system-storage PVC block.

For example:

- apiVersion: "v1"
  kind: "PersistentVolumeClaim"
  metadata:
    name: "system-storage"
  spec:
    accessModes:
      - "ReadWriteMany"
    resources:
      requests:
        storage: "100Mi"
    storageClassName: efs

(1) https://raw.githubusercontent.com/3scale/3scale-amp-openshift-templates/2.2.0.GA/amp/amp.yml

How

• Add new parameter
• Set default value to be empty (see below example)

cat /tmp/pvc.yaml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
 name: pvc-engineering
spec:
 accessModes:
  - ReadWriteMany
 resources:
   requests:
     storage: 10Gi
 storageClassName: