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/