Revert default to pulling images

This reverts merge request !228 (merged).

Unconditionally doing --pull turns out to be a bad idea. For a local build of the nso-docker images, the images are of course locally available, yet with --pull, docker build will try to pull them from.. somewhere, which fails and then the whole build fails.

pull access denied for cisco-nso-dev, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

so I guess we need to figure out a different way of accomplishing this. Reverting for now.