Enable System set_variable resource in DS API

@sergii-tkachenko Can you review?

Enables the built-in Services `set_variable` resource to allow other apps to authenticate and then set a Drupal variable.

Removes `dosomething_user_set_member_count` function as this variable will only be set via the API (or could just be set via `variable_get`)
Add a whitespace before `@todo` and remove one before `Need` :bowtie: