Add helm values for Logging `ImagePullPolicy`

Currently imagePullPolicy is not set for the elastic and kibana CRs. We should be able to set via spec.podTemplate.spec.imagePullPolicy in kibana and elastic.

We should add this value and set it to a helm value (i.e. something like {{ .Values.kibana.image.pullPolicy }}/{{ .Values.elasticsearch.image.pullPolicy }}) with a default of IfNotPresent and then document these defaults/overrides in the table here.

AC:

• Logging CRs updated to allow for overriding the pull policy via helm values
• Ways to override the pull policies added/documented in the table here