Allow to set hostnetwork on Webhook

[mcantinqc]

Discussed in #462

Allow to use Webhook with the argument hostnetwork: true.

Use-Case

To address limitations faced on EKS with custom CNIs such as Calico, Weave, or Cilium, we require the ability to use webhooks with the hostnetwork: true argument. More details on this can be found in the following article: https://medium.com/@denisstortisilva/kubernetes-eks-calico-and-custom-admission-webhooks-a2956b49bd0d.

Specification

• Enable the setting of hostnetwork on the webhook. You can find the relevant code in the Keda chart's values.yaml file here: https://github.com/kedacore/charts/blob/main/keda/values.yaml#L80.
• (Optional) Allow customization of the webhook port. This would require adding a new argument to the Keda, https://github.com/kedacore/keda/blob/0e7662d0fcb6d8084f2161ba70b9d457442f80f4/cmd/webhooks/main.go#L84.