Utilize global imagePullPolicy for Loki

Since we now have a global imagePullPolicy value, we should make sure this is passed to the proper places in Loki to affect its pull policy.

We should also update the document here to list out the overrides if someone wants to set a different pull policy for Loki.

AC:

• Loki uses the global pull policy for all containers
• Docs updated to show ways to override Loki container pull policies

Helpful tips:

• Start by checking the loki values file and searching for pullPolicy or imagePullPolicy. Note all values locations for this.
• In the BB loki values add those values and set their value to {{ .Values.imagePullPolicy }} so that they inherit the global (example for jaeger)
• Add those values in a new table row to the table here
• Deploy with loki enabled, imagePullPolicy: Always in your values, and validate that all containers have an imagePullPolicy set to Always.