Add ability to specify custom python requirements.

This lets you specify additional python packages to be installed
into the edxapp virtual environment.

The `EDXAPP_EXTRA_REQUIREMENTS` should be a list of dict objects
with `name`, `version` (optional) and `extra_args` (optional) keys,
that correspond to the options of ansible's pip module.

Example:

    EDXAPP_EXTRA_REQUIREMENTS:
      - name: mypackage
        version: 1.0.1
      - name: git+https://git.myproject.org/MyProject#egg=MyProject
adding libfreetype6-dev
Merge pull request #1540 from edx/han/libfreetype6-dev

adding libfreetype6-dev to release
New base AMI, updating AMIs for Ubuntu
Merge pull request #1563 from edx/han/release-sandbox-ami

New base AMI, updating AMIs for Ubuntu
Use boto 2.32.1
replace geturl with a shell out to curl - to fix broken certificate management
Merge pull request #1631 from edx/han/release-fix-geturl

replace geturl with a shell out to curl - to fix broken certificate mana...
Add the new version of apt.
validate certs=no on apt_repository due to ansible bug
Switch to using the new apt_repository module.
Merge pull request #1645 from edx/derf/ignore_cert_validations

Derf/ignore cert validations
Don't restart edxapp after updating on devstack

- when using `/edx/bin/update edx-platform (branch)`
Added Brandon DeRosier to authors
Add sane defaults for djfs.

Conflicts:
	playbooks/roles/edxapp/defaults/main.yml
Merge pull request #1675 from edx/feanil/djfs_defaults_release

Add sane defaults for djfs.
Update to use the stable ppa for datadog.

Remove the old repository.

Explicitly control the version of Datadog.
Merge pull request #1676 from edx/feanil/update_datadog_agent

Feanil/update datadog agent
quick fix to issue-certificate
Merge remote-tracking branch 'origin/release' into feanil/release_into_master

Conflicts:
	playbooks/roles/datadog/tasks/main.yml
	playbooks/roles/rabbitmq/tasks/main.yml