Dan/Sneha making extra_disks details available to preamble

preamble mounts the extra disks hence it should know abt extra disk
details.
this functionality was broken during launch.rb refactoring. bringing it
back.
seperated orchestration logic from vm
Expose underlying fog VCloudDirector object

As a consumer of vcloud-tools if I have instantiated a FogModelInterface
let me access the underlying vcloud object.