Gns 81

Added two relevant methods to `RackspaceCtrl` class:
 * `_get_shared_images` to perform HTTP requests against gns3-ias server
 * `list_images` higher-level method to retrieve a dictionary (image_id, image_name) of shared images ready to be used by the GUI client

A note about integration tests: I've made significant changes to give developers the opportunity to pass rackspace credentials through command line and have more control about which tests to run and how.
More details here: https://planctechnologies.atlassian.net/wiki/display/GNS/Developer+Installation