Refactor stub implementation of LTI Provider. BLD-601.

CHANGELOG.rst

common/djangoapps/terrain/start_stubs.py

common/djangoapps/terrain/stubs/http.py

common/djangoapps/terrain/stubs/lti.py

common/djangoapps/terrain/stubs/start.py

common/djangoapps/terrain/stubs/tests/test_lti_stub.py

common/lib/xmodule/xmodule/lti_module.py

common/lib/xmodule/xmodule/tests/test_lti_unit.py

lms/djangoapps/courseware/features/lti.py

lms/djangoapps/courseware/features/lti_setup.py

lms/djangoapps/courseware/mock_lti_server/__init__.py

lms/djangoapps/courseware/mock_lti_server/server_start.py

...goapps/courseware/mock_lti_server/test_mock_lti_server.py

lms/djangoapps/courseware/tests/test_lti_integration.py

lms/envs/acceptance.py

lms/envs/devstack.py