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/
Fix some URLs and text in release emails to include "-incubating" where appropriate.

Reviewed at https://reviews.apache.org/r/28449/
Don't intercept methods on thrift that don't return Response.

Testing Done:
./gradlew -Pq build

Bugs closed: AURORA-952

Reviewed at https://reviews.apache.org/r/28605/
Minimize the state consumed when collecting preemption victims.

Bugs closed: AURORA-121

Reviewed at https://reviews.apache.org/r/28572/
Modify  TRequestsTransport to raise an exception 4xx or 5xx responses.

This changes TRequestsTransport to raise an exception on 4xx or
5xx responses. Previously the TRequestsTransport would ignore those
responses and treat the response as valid. The response would not be
valid JSON and cause thrift decoding errors later on.

Testing Done:
./pants src/test/python/apache/aurora/common:test_transport -v

Bugs closed: AURORA-949

Reviewed at https://reviews.apache.org/r/28451/
Improve unbounded ref exception.

This improves the contents of the exception that is raised when the
executor is given a TaskConfig with unbounded refs.

Testing Done:
./pants src/test/python/apache/aurora/executor/common:task_info

Bugs closed: AURORA-939

Reviewed at https://reviews.apache.org/r/28483/
Use requirements.txt for 3rdparty/python.

Also drop an unused Flask dependency and upgrade CherryPy to 3.6.0
(which has a functioning version on PyPI and thus doesn't need a
custom repository set in the BUILD file).

Testing Done:
./pants src/test/python:all

Bugs closed: AURORA-617

Reviewed at https://reviews.apache.org/r/28610/
Reject jobs containing an empty cron schedule.

Bugs closed: AURORA-424

Reviewed at https://reviews.apache.org/r/28571/
Prevent Aurora from creating tasks with 0 disk.

Bugs closed: AURORA-956

Reviewed at https://reviews.apache.org/r/28621/
Add a caching ClusterState implementation.

Bugs closed: AURORA-121

Reviewed at https://reviews.apache.org/r/28607/
Reduce minimum branch coverage requirement to avoid flakiness.

Reviewed at https://reviews.apache.org/r/28660/
Remove getVersion RPC and DEPRECATEDversion Response field.

Bugs closed: AURORA-143, AURORA-467

Reviewed at https://reviews.apache.org/r/28623/
Collect HTTP stats on every request.

Reviewed at https://reviews.apache.org/r/28728/
Remove redundant storage.write.

Reviewed at https://reviews.apache.org/r/28725/
Drop custom thrift doc generator in favor of the one included in thrift.

Bugs closed: AURORA-947

Reviewed at https://reviews.apache.org/r/28682/
Remove Response.messageDEPRECATED field.

Bugs closed: AURORA-466

Reviewed at https://reviews.apache.org/r/28674/
Make abstract method annotations on ConfigurationPlugin effective.

Reviewed at https://reviews.apache.org/r/28693/
Remove unused DefaultServlet subclass.

Reviewed at https://reviews.apache.org/r/28738/
Reorganizes documentation README and updates links to current documentation pages.

Reviewed at https://reviews.apache.org/r/28613/
Don't fall back to old command syntax in the new client.

Bugs closed: AURORA-782

Reviewed at https://reviews.apache.org/r/28771/