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 #1035 from mtyaka/extra-requirements

Add ability to specify custom python requirements.
wip

bug fix

rename

Adding renamed

Adding key

typo

bad arg

Adding force yes

nuclear

comments, vars

ignore errors

fixing install check

ensuring started

moving tasks in, conditional inc not working

changing tac

missed one

typo

fixed conditional