Download sky_snapshot from the cloud

This adds logic to download and use the sky_snapshot binary from
Google cloud storage when running the 'sky_tools build' command.
The downloaded binary is put into lib/cache/... The binary is
chosen to match the REVISION in the sky_engine package in the
packages directory of whichever package the user wishes to
build a flx from.

Known issues:
*) Assumes linux-x64 host
*) Assumes download will always produce valid executable
*) No clearing of stale cache entries