JCLOUDS-224: Allow pagination when retrieving the list of users belonging to an Enterprise

Changes suggested in the Pull Request
JCLOUDS-198: Implemented the PagedIterable in Abiquo

* Refactored all the domain objects to return Iterables
  instead of Lists, to allow a more flexible approach when
  using the PagedIterable.
* Added a generic PaginatedCollection that knows how to advance
  any paginated collection.
* Changed all methods from the Abiquo APIs that allow pagination
  to return the corresponding PagedIterables and PaginatedCollections.
* Added Expect tests for every paginated resource.
* Handle the special case of user pagination, to bypass Abiquo issue
  http://jira.abiquo.com/browse/ABICLOUDPREMIUM-5927
JCLOUDS-205: Do not create duplicated groups in Abiquo

Refactored the AbiquoComputeServiceAdapter to avoid creating
more than one group with the same name, when creating multiple
nodes concurrently.