cv_configlet : implement diff and set changed to true when appropriate in check mode

[b-abadie]

Fixes #120

[titom73]

Hi @b-abadie

Could you please comment with what this PR changes in inputs and outputs ?

Regards,

[b-abadie]

Sure.
This PR does not change what input cv_configlet expects.

It does change what cv_configlet outputs : if one or more configlets have been modified, the modules outputs an additional key, diff, that will be interpreted and displayed by Ansible. This diff is of type prepared (see here)
This text outputs is actually a concatenation of all configlets' diffs, as we can update more than one in a single cv_configlet execution.

Note that my patch leverage the existing call to compare(), i.e. the diff was generated but not used verbatim.

Additionally, the PR changes the behavior of cv_configlet : when in check mode (dry run), the module now returns changed as true if any configlet would have been added, deleted or modified. If I recall correctly (this change was sitting in a branch of mine), the diff is not displayed by Ansible when the task has changed set to false (i.e. green in ansible-playbook).
But even without the diff, it seems to me that this behavior is preferred, as I stated in #120 .