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
Merge pull request #1678 from edx/feanil/rc-lavash-0

Feanil/rc lavash 0
Operationalize harprofiler.
move venv to subdir and activate it.
Merge pull request #1681 from edx/benp/harprofiler

Operationalize harprofiler.
Harprofiler repo is now under edx. Disable test cruft.

The test was under dev and shouldn't have been merged. (This
approach is incompatible with ansible's design.)
Merge pull request #1684 from edx/benp/move-harprofiler-ref

Harprofiler repo is now under edx. Disable test cruft.
Compiling Insights Translations
Merge pull request #1685 from edx/insights-translations

Compiling Insights Translations
Revert "Compiling Insights Translations"
Merge pull request #1686 from edx/revert-1685-insights-translations

Revert "Compiling Insights Translations"
Gather facts so the security role succeeds
Merge pull request #1689 from edx/will/gather-facts-for-security-role

Will/gather facts for security role
Merge pull request #1035 from mtyaka/extra-requirements

Add ability to specify custom python requirements.
harprofiler ami creation via packer.

TE-583
Merge pull request #1687 from edx/benp/packerify-harprofiler

harprofiler ami creation via packer.
Merge pull request #22 from edx/master

Try to merge  Master@2014.11.11