Teach buildbot to shutdown EC2 slaves when idle

[brson]

Would be so much better if we were not burning through CPU hours when there are no builds happening. AFAICT the EC2LatentBuildslave does not support this so may require upstream buildbot hacking.

Depends on #17852 to avoid lots of build failures.

[brson]

Looks like we just need to set build_wait_timeout: http://docs.buildbot.net/current/manual/cfg-buildslaves.html#latent-buildslaves

[brson]

Huh, we are currently setting build_wait_timeout to something very large (3 days), presumably to work around a bug. Not sure.

[brson]

Done.