CYB-327 thor keys:lti_new should not give error if there's no API key

  * The thor task to generate LTI keys prints out the new keys and the API key,
     if there is no API key it was trying to concatenate to nil

https://jira.ets.berkeley.edu/jira/browse/CYB-327
Merge pull request #198 from coyote/setup_new_server_without_api_key

CYB-327 thor keys:lti_new should not give error if there's no API key