Add release namespace to manager WATCH_NAMESPACE env when watchNamesp…

[fouadsemaan]

…ace specified

Addresses issue #748

Include the release namespace in the WATCH_NAMESPACE env variable when .Values.watchNamespace is non-empty.

This is consistent with creating rolebindings when .Values.watchNamespace is set.

Checklist

• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)
• [x] README is updated with new configuration values (if applicable) learn more
• [x] A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #