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>