AURORA-214: Make sandboxes safer.

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

Bugs closed: AURORA-204

Reviewed at https://reviews.apache.org/r/17913/
Moving RUN_OVERLAP check into receiveJob() to prevent failure on startup.

Bugs closed: AURORA-210

Reviewed at https://reviews.apache.org/r/17955/
Implementing support for DRAINING state.

Bugs closed: AURORA-82

Reviewed at https://reviews.apache.org/r/17723/
Fixing python test failure.

Reviewed at https://reviews.apache.org/r/18086/
Added rollback_on_failure to UpdateConfig

Bugs closed: AURORA-148

Reviewed at https://reviews.apache.org/r/17645/
Added LeaderRedirectFilter to scheduler home page.

Testing Done:
gradle clean build
tested on localhost, verified behavior via log statements.

Bugs closed: AURORA-211

Reviewed at https://reviews.apache.org/r/18036/
Adding "sla get_task_up_count command".

Bugs closed: AURORA-206

Reviewed at https://reviews.apache.org/r/18042/
Cron predictor conformance test.

Quartz doesn't support "standard" cron syntax (as described in
crontab(5)[1]) and deviates [2] in a lot of ways that would lock us into
it. Some of the ways it varies include:
  * '00' is not a valid minute
  * day-of-week is 1-indexed instead of 0-indexed
  * it supports a seconds field, which is an unrealistic constraint for aurora to deliver
  * the common "* * * * *" pattern is invalid

Rather than adopt Quartz's syntax I propose introducing a translation
layer that will allow users to write in the more familiar crontab
syntax. This change adds test data that can be used as a conformance
test - a valid CronPredictor implementation must both accept the same
syntax and make the same predictions as those referenced here. In a
future review I will introduce a Quartz cron subsystem that passes this
test.

[1] http://unixhelp.ed.ac.uk/CGI/man-cgi?crontab+5
[2] http://quartz-scheduler.org/documentation/quartz-2.2.x/tutorials/tutorial-lesson-06

Testing Done:
./gradlew build

Bugs closed: AURORA-132

Reviewed at https://reviews.apache.org/r/18149/
AURORA-170: expose process exit status via observer UI