Add ability to retry image pulling

Sometimes, the registries may intermittently fail to deliver the
images. This is often seen in the CI, though it also happens with
production deployments, even those with internal registries and/or
registry mirrors - due to sheer load when trying to pull the
images from many hosts.

This patchs adds two new vars to control retry behaviour.
The default has been set to make users happier by default. :-)

Change-Id: I81ad7d8642654f8474f11084c6934aab40243d35