ceph: remove nullable attribute from pre-1.16 schema

The nullable attribute was causing the integration tests to fail
on 1.13 and earlier where it is not supported in the schema.

Signed-off-by: Travis Nielsen <tnielsen@redhat.com>

cluster/charts/rook-ceph/templates/resources.yaml

@@ -1288,7 +1288,7 @@ spec:
               x-kubernetes-preserve-unknown-fields: true
       subresources:
         status: {}
----    
+---
 {{- else }}
 ---
 apiVersion: apiextensions.k8s.io/v1beta1
@@ -1432,7 +1432,6 @@ spec:
                 storageClassDeviceSets: {}
             driveGroups:
               type: array
-              nullable: true
               items:
                 properties:
                   name:

cluster/examples/kubernetes/ceph/pre-k8s-1.16/crd.yaml

@@ -158,7 +158,6 @@ spec:
                 storageClassDeviceSets: {}
             driveGroups:
               type: array
-              nullable: true
               items:
                 properties:
                   name:

tests/framework/installer/ceph_manifests.go

@@ -1508,7 +1508,6 @@ spec:
                 storageClassDeviceSets: {}
             driveGroups:
               type: array
-              nullable: true
               items:
                 properties:
                   name: