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
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
Add DJFS settings (OSPR-50)

Add DJFS settings (OSPR-50)
To solve this pr:
https://github.com/edx/edx-platform/pull/5291
Handle aspen.1 openedx release
Use curl instead of get_url

* Handles cert validation from behind a proxy better
* Also this commit updates build server validation (i.e. testcase
    paths) that have changed on platform
Merge pull request #1668 from edx/benp/fix-packer-cert-error

Use curl instead of get_url for packer zip download
Merge pull request #1669 from edx/handle-aspen.1-tag

Handle aspen.1 openedx release
Don't restart edxapp after updating on devstack

- when using `/edx/bin/update edx-platform (branch)`
Added Brandon DeRosier to authors
Add HAProxy check user to prevent `max_connect_error` exceptions
Merge pull request #1673 from mitodl/cg/add_haproxy_mariadb_user

Add HAProxy check user to prevent `max_connect_error` exceptions