[CLOUD-1981] docker image update script runs in noop mode

[mihaibuzgau]

The --noop flag tells Puppet to determine which resources are out of sync, and to report them without actually synchronizing them. The 'onlyif' and 'unless' commands of an Exec are used in the process of determining whether the Exec is already in sync, therefore they must be run during a --noop Puppet run.

This PR changes the 'onlyif' command for ensure == 'latest' or $image_tab == 'latest' from a script that takes action (downloads images) to a command that only check the state.

Fixes: #241

[davejrt]

LGTM

[coder-hugo]

This change makes the value latest for the ensure of a docker::image useless since the $image_install is only executed if the image in not already installed (unless => $_image_find). So you will never get a never version of the specified installed unless you change the image_tag

[davejrt]

@coder-hugo we are aware this was a breaking change. This was reflected in the documentation as well as in the version numbers, and we discussed are reasons in issue #316.

[coder-hugo]

IMHO such a breaking change should be noted within the changelog and not somewhere hidden in the README. I'd expect that most user will just have a look to the changelog before updating a module and they will not read through the whole README to find some breaking changes.

[davejrt]

We'll take your feedback on board for any future breaking changes.