Fix backend.OSEnvironment be/hv parameters

Commit 67fc3042 added some more
variables to be exported to OSEnvironment, but it has two bugs:
  - wrong variable name (env vs. result)
  - in OSEnvironment we don't have the automatic converstion to strings
    that we do in hooks, so we must manually enforce this

With this patch instance creations work again.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>