ec2 scripts: optionally support small instances

There are 2 types of HVM AMIs on Amazon:

1. Windows based that support all instances types
2. Linux based that support instances larger than medium only

Linux based AMIs suit OSv better because their default seciurity
policy exactly fits OSv model (SSH access) and doesn't require
reconfiguration.

Windows based AMIs require default security policy reconfiguration
in order to allow SSH access but from other hand allow running OSv
on free-tier eligible micro instances which is good for enthusiasts
and initial evaluations.

In order to support both options this patch extends release scripts
with ability to choose between Windows and Linux based templates
during AMIs creation.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>