Allow ECK-Operator to easily watch deployed namespace

[developer67890]

This change adds a Helm Chart value singleNamespaceMode which configures to the operator to watch its deployed namespace if true.

While this could have been achieved by including the deployed namespace in the managedNamespaces list, it requires a user to specify a second time this namespace.

helm install elastic-operator ./eck-operator -n my-namespace --create-namespace \
  --set=managedNamespaces='{my-namespace}' \
...

This model is fragile and prone to mistyping or omissions. Other products have accomplished a similar feature by including something like clusterWide=false, which will result in the same behavior this PR achieves.

Changes:

• add singleNamespaceMode value to values.yaml, including short comment explanation
• if singleNamespaceMode = true, only include .Release.Namespace in the namespaces field of the Configmap.
• update documentation for "Install the operator restricted to a single namespace"

Resolves #8828

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
646dc1e, 56ee0ca, 13220a3

Please, read and sign the above mentioned agreement if you want to contribute to this project

[prodsecmachine]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)