Code hygiene: mark _private stuff so
BlockDeviceMapping for RunInstance and SpotRequests

In AWS DeleteOnTermination defaults to True. In boto it defaults to
False. Here, we side with AWS with the intention of simplifying the
disposable instances use case.

On a tangentially related note, AWS doesn't require a device name, while
boto does. However, in auto-scaling groups device names are required by
AWS. For now, hc2000's schema requires device names as well.
BlockDeviceMappings for AutoScaling groups

Technically, for launch configurations.

Boto is currently buggy in the way it handles block device mappings in
launch configurations, so workaround it. Here, this is doen by tricking
boto into letting plain AWS API parameters through.

(See https://github.com/boto/boto/pull/1574 for details on the boto
issue, or a fix)