Support pull-if-not-present resource image pull policy

[arikkfir]

Currently, Deployster supports either:

• always pull resource images (default)
• relying on Docker to pull the image if not already present (includes latest which is unfortunate)

It would be beneficial to support a new --pull-policy argument with the following values:

• always which would always [re]pull resource images regardless if they are present or not (esp. latest)
• if-not-present which would always pull the latest tag, but avoid pulling tags if they are already present
• never which would never pull new tags (failing if a tag is not already present in the system)
  • is this useful?

The default IMO should be if-not-present.

[arikkfir]

Also, just noticed that currently Deployster tries to pull resource images per resource - meaning that if two resources use the same image, it's downloaded twice. This should be fixed as part of this issue.