Replace SEGMENT_IO_LMS & SEGMENT_IO_LMS_KEY with single key SEGMENT_KEY

cms/envs/aws.py

cms/envs/bok_choy.env.json

cms/envs/common.py

cms/envs/dev.py

cms/envs/test.py

cms/envs/yaml_config.py

cms/templates/widgets/segment-io.html

common/djangoapps/student/models.py

common/djangoapps/student/views.py

common/djangoapps/third_party_auth/pipeline.py

common/djangoapps/track/shim.py

common/djangoapps/track/views/segmentio.py

common/djangoapps/track/views/tests/test_segmentio.py

lms/djangoapps/courseware/tests/test_views.py

lms/djangoapps/courseware/views.py

lms/djangoapps/shoppingcart/models.py

lms/djangoapps/shoppingcart/tests/test_models.py

lms/djangoapps/verify_student/tests/test_views.py

lms/djangoapps/verify_student/views.py

lms/envs/aws.py