tests/k8s: add new_pod_config() to lib.sh

Copied the new_pod_config() and pod-config.yaml.in from CCv0 branch
tests' integration/kubernetes/confidential/tests_common.sh and fixtures.
Unlike the original version, new_pod_config() now gets the runtimeclass
by parameter as the RUNTIMECLASS environment variable seems not broadly
used on main branch's CI.

The pod-config.yaml.in was changed as the diff shows below. In
particular the imagePullSecrets was removed to avoid it throwing a
warning on the pod's log.

```
--- a/tests/integration/kubernetes/runtimeclass_workloads/pod-config.yaml.in
+++ b/tests/integration/kubernetes/runtimeclass_workloads/pod-config.yaml.in
@@ -5,12 +5,10 @@
 apiVersion: v1
 kind: Pod
 metadata:
-  name: busybox-cc
+  name: test-e2e
 spec:
   runtimeClassName: $RUNTIMECLASS
   containers:
-  - name: nginx
+  - name: test_container
     image: $IMAGE
-    imagePullPolicy: Always
-  imagePullSecrets:
-  - name: cococred
\ No newline at end of file
+    imagePullPolicy: Always
\ No newline at end of file
```

Co-authored-by: Georgina Kinge <georgina.kinge@ibm.com>
Co-authored-by: Megan Wright <Megan.Wright@ibm.com>
Co-authored-by: stevenhorsman <steven@uk.ibm.com>
Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>

tests/integration/kubernetes/runtimeclass_workloads/pod-config.yaml.in

@@ -0,0 +1,14 @@
+# Copyright (c) 2021, 2022 IBM Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+apiVersion: v1
+kind: Pod
+metadata:
+  name: test-e2e
+spec:
+  runtimeClassName: $RUNTIMECLASS
+  containers:
+  - name: test-container
+    image: $IMAGE
+    imagePullPolicy: Always