Fix :pull_if_missing behavior

It turns out chef-docker does not support using `docker_image` resource names that include tags.  This PR uses explicit repository and tag fields which fix broken `:pull_if_missing` behavior.

Note different behavior depending on `node.deis.dev.mode` attribute:

 * Dev mode `true` - :pull_if_missing will be used so as not to overwrite existing images
 * Dev mode `false` - :pull will be used to facilitate automatic upgrades