4.12 backport: Multiple extra manifests in the same file are not applied correctly

Custom manifest files can be placed in the /openshift folder so that they will be applied during cluster installation.
Anyhow, if a file contains more than one manifests, all but the first are ignored.

Always

1.Create the following custom manifest file in the /openshift folder:

```
apiVersion: v1
kind: ConfigMap
metadata:  
  name: agent-test  
  namespace: openshift-config
data:  
  value: agent-test
---
apiVersion: v1
kind: ConfigMap
metadata: 
name: agent-test-2
namespace: openshift-config
data: 
  value: agent-test-2
```
2. Create the agent ISO image and deploy a cluster

ConfigMap agent-test-2 does not exist in the openshift-config namespace

ConfigMap agent-test-2 must exist in the openshift-config namespace