Added soft shutdown, simplified reboot process

- removed unused param from find_and_terminate_instance
- added clean shutdown attempt for both reboot and terminate
- obtaining domain XML from libvirt and using it on reboot,
  thus avoiding having to re-attach EBS volumes

Fixing EUCA-5771, EUCA-4332, EUCA-5993
Merge branch 'testing' of git+ssh://git.eucalyptus-systems.com/eucalyptus into dev/dmitrii/shutdown
Merge branch 'dev/dmitrii/shutdown' into testing