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
Modify edxlocal role to be able to create databases on RDS.

The database creation scripts in the edxlocal role only work if MySQL
is hosted locally. Letting ansible take care of creating the databases
is very convenient, so modify the script to be able to create
databases on remote servers.