Always try to pull the latest Docker images

This should not impact those Docker images that are immutable, but it
should aid in those deployments that want to use the mutable docker
image tags.

For example:

timescaledev/timescale/timescaledb-ha:pg12-ts1.7     is   mutable
timescaledev/timescale/timescaledb-ha:pg12.3-ts1.7.1 is immutable

charts/timescaledb-single/values.yaml

@@ -21,7 +21,7 @@ image:
   # https://github.com/timescale/timescaledb-docker-ha
   repository: timescaledev/timescaledb-ha
   tag: pg11-ts1.7
-  pullPolicy: IfNotPresent
+  pullPolicy: Always
 
 # These secrets should exist before the Helm is used to deploy this TimescaleDB.
 # You can use generate_kustomization.sh to help in creating these secrets, or have
@@ -353,7 +353,7 @@ prometheus:
   image:
     repository: wrouesnel/postgres_exporter
     tag: v0.7.0
-    pullPolicy: IfNotPresent
+    pullPolicy: Always
   # Extra custom environment variables for prometheus.
   # These should be an EnvVar, as this allows you to inject secrets into the environment
   # https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.16/#envvar-v1-core