Inform the OS create script of reinstalls

Sometimes reinstalls are slightly different than new installs. For
example certain partitions may need to be preserved accross reinstalls.
In order to do that on a per-os basis we pass in the INSTANCE_REINSTALL
variable to inform the create script about when a reinstall is
happening.

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