Ref #570: do not hard code secret in integration test

[leplatrem]

#570

Replace hard coded secret with something like this:

$ ctms/bin/client_credentials.py -e master@local.host integration-test -o creds.json
$ CLIENT_SECRET=$(jq .secrets creds.json)

And (for example)

$ echo "CTMS_CLIENT_SECRET=$CLIENT_SECRET" >> basket.env