Move thermos_runner out of the apache.aurora.executor package.

The drawback of having it in the apache.aurora.executor package is that if
we want to build a thermos_runner binary from the apache.aurora.executor
distribution, it transitively pulls in mesos.native, making the
thermos_runner pex like 70MB.  thermos_runner.pex really only has
dependencies within apache.thermos and the thermos_runner exit-code contract
is a bit more clear now, so putting it into apache.thermos makes a bit more
sense now.  it also makes thermos_runner.pex 566k instead of 70MB.  it also
fixes AURORA-823.

Testing Done:
build-support/jenkins/build.sh

Bugs closed: AURORA-823

Reviewed at https://reviews.apache.org/r/28345/