using serial var for all plays
makes an all-caps var for celery workers
adding libfreetype6-dev
Merge pull request #1539 from edx/han/libfreetype6-dev

adding libfreetype6-dev
Change the sudo permissions for sandbox.

Instead of rm directly we now do a find.
git checkouts over ssh need ~/.ssh to exist
Merge pull request #1542 from edx/feanil/update_sandbox_sudoers

Change the sudo permissions for sandbox.
Merge pull request #1522 from edx/jarv/add-serial-to-plays

using serial var for all plays
Don't run pre-supervisor on abbey builds.
Merge pull request #1543 from edx/feanil/quiet_abbey_builds

Don't run pre-supervisor on abbey builds.
adding changelog
Merge pull request #1531 from edx/jarv/make-celery-workers-override

makes an all-caps var for celery workers
this needs to default to false because it currently depends on
analytics-server

analytics-server is a private repo
Merge pull request #1547 from edx/jarv/turn-off-analytics-by-default

this needs to default to false because it currently depends on
pip installs are hanging for xqueue

We were hanging on this:

```
Obtaining pika from
git+https://github.com/pika/pika.git@a731347fc0#egg=pika (from -r
/edx/app/xqueue/xqueue/requirements.txt (line 14))
  git clone in /edx/app/xqueue/venvs/xqueue/src/pika exists with URL
git://github.com/pika/pika.git
  The plan is to install the git repository
https://github.com/pika/pika.git
What to do?  (s)witch, (i)gnore, (w)ipe, (b)ackup
```
Merge pull request #1549 from edx/jarv/exists-actions-w-for-xqueue

pip installs are hanging for xqueue
TRACKING_SEGMENTIO_WEBHOOK_SECRET
TRACKING_SEGMENTIO_WEBHOOK_SECRET defaults
Merge pull request #1550 from edx/han/TRACKING_SEGEMENTIO

TRACKING_SEGMENTIO_WEBHOOK_SECRET
Adding new env config value