Gns-14 - Populate server images to start instances from

This code change now uses gns3ias to populate a list of server images in the preferences pane.  Future code changes will actually start instances using the selected image.

I did notice a poor interaction between the script that creates server images and the version number that the gui includes when making a request to the image access server.  The version numbers mismatch (something like "a" vs "alpha") and the request to share images shows no matching images.  I will create an issue for this.  In the meantime, I renamed the image in Rackspace by booting up an instance and creating a new snapshot with a matching name.