Add options for installing and configuring a CAS attribute mapper

This is to go along with https://github.com/edx/edx-platform/pull/2948 

It just installs a module if one is specified and adds the attribute hash.  A fully baked CAS config would be something like:
```yaml
EDXAPP_CAS_SERVER_URL: "https://example.com/cas"
EDXAPP_CAS_EXTRA_LOGIN_PARAMS:
  app: "edx-platform"
  api-key: "blahblah"
EDXAPP_CAS_ATTRIBUTE_PACKAGE: "git+https://github.com/mitocw/mitx_cas_mapper"
EDXAPP_CAS_ATTRIBUTE_CALLBACK:
  module: "mitx_cas_mapper"
  function: "populate_user"
```