We changed the imagePullPolicy to IfNotPresent so that user can easily
install Longhorn in air-gap instalation.
Also add a bash script for the developer to quickly change all the
imagePullPolicies back to Always so that k8s always pull the
lastest images. This will be useful when dev use the tag such as master.
Longhorn #1491
Signed-off-by: Phan Le <phan.le@rancher.com>