Core - Provide global value for `imagePullPolicy` - defaulted to `IfNotPresent`

Related to #938 (closed) .

Following the information documented in the imagePullPolicy table, create a global value for imagePullPolicy that will pass through to all packages.

Example:

Istio Controlplane Values:

# Under {{- define "bigbang.defaults.istio" -}}
imagePullPolicy: {{ .Values.imagePullPolicy }}

AC:

• Create a new global value in chart/values.yaml for imagePullPolicy, set with a default of "IfNotPresent"
• Ensure this value is passed down to all core packages at the correct values spec. Use the example above, for packages with multiple values make sure it is passed to all spots needed.
• Update the doc with (1) the new override value and how to use it in the ImagePullPolicy at Big Bang Level section and (2) a brief comment under the ImagePullPolicy per Package section to note that any of the value can be used per package to override the global set.