Add an "overflow" ProcessIncoming autoscale group

Set up an autoscale group that will kick in if the backlog gets even longer than
the trigger for the normal ProcessIncoming scale-up threshold. This should help
in the case where our spot price bid is too low for extended periods of time.
Use hvm or pv images as needed. Update AMIs.

Turns out m1.xlarge instances can't use hvm images, so we re-deploy with pv
images (and update the webserver image while we're at it).
Send stdout and stderr to a separate log file

Helpful for debugging.
Add ami-creation config files.