Add variable image.pullSecret to Helm chart to support specifying an
image pull secret. This is mostly useful for people running an in-house
fork of cert-manager-webhook-dnsimple.
When using a GitOps deployment model, these resources are always
considered "out of sync", as the resulting Certificate object's
'duration' field also specifies minutes & seconds.
These changes are essentially a no-op, but match the object, for
GitOps compatability.