httpserver: document /os/cmdline

Be more explicit about what we are doing

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
release-ec2: introduce AWS region/zone/placement group parameters

These parameters define created AMIs/instances placement,
useful for performance tests where instances locality matter.

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