Canada SSO: fixes users address country.

#### What's this PR do?
- Changes user's address on DS Canada so new users created during login and registration have 'CA' in country field
- Changes `dosomething_user`: `_dosomething_user_register_display_fields` now doens't unset address, but restrict access to it 

#### How should this be manually tested?
- Run tests after cloning this branch
- Test that functionality works ad before after `dosomething_user` change: `ds test default`
  ![screen shot 2014-10-23 at 6 42 11 pm](https://cloud.githubusercontent.com/assets/672669/4770188/0d3b4eb8-5b83-11e4-91e1-1ee624861772.png)
- Enable and setup DoSomething  Canada module
- Test Canada-specific functionality: `ds test canada`
  ![screen shot 2014-10-23 at 6 44 29 pm](https://cloud.githubusercontent.com/assets/672669/4770218/716cca9c-5b83-11e4-964b-58cce419e45e.png)

#### What are the relevant tickets?
Fixes #3217 — Save CA as address country for DS Canada User Registration
There is a `dosomething_user_address_country` variable which I added in #3213.  I suppose it doesn't hurt to have a constant here, but it feels kinda duplicated...