Correct naming of tasks in floating ip migration flows

Adhere to common naming scheme:

* '-retrieve'-suffixed attributes added to store manually to keep IDs of objects
  to migrate;
* Retrieve tasks require '-retrieve' attributes and provide '-binding'
  attributes;
* All tasks are named after name of what they provide.
Exception from naming scheme for Image resources

Image resource doesn't have distinguished Retrieve task. Thus, it deviates from
standard naming scheme (see previous commit).