Refactors CanadaSSOClient.

- Uses drupal_http_request instead of curl
- Moves corresponding object variables to class constants
- Replaces Exceptions with silent logging
Deprecates CanadaSSOClient->getRemoteUser().
Canada: Fixes remote users creation for local admins.
Merge pull request #3534 from sergii-tkachenko/canada-refactor-sso-client

Canada refactor sso client