Update fixed_ip_associate to not use relationships
Update fixed_ip_associate to not use relationships

 * fixed_ip_associate was using the FixedIp.instance and FixedIp.network
   relationships, which were removed.  This change updates the method to
   use instance_id and network_id instead.
 * Adds unit tests for fixed_ip_associate
 * Resolves bug 943029

Change-Id: I0829d110058b8799a520ab68b5fa1b2370b9cc3e

n0ano/ganttclient