Parameterize forensics-system namespace

[shaoxt]

Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE REQUEST
What happened:
kube-forensics controller and its worker may run in different namespace. So need to make it as an argument.

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• kube-forensics version
• Kubernetes version :

$ kubectl version -o yaml

Other debugging information (if applicable):

• PodCheckpoint status:

$ kubectl describe -n forensics-system podcheckpoint <checkpoint-name>

• controller logs:

$ kubectl logs -n forensics-system <forensics-controller-manager pod>