Allow plain text for credentialsId field.

instead of doing this: 
credentialsId: 'my_cred_id' 

I could do:

credentialsId: '{"login_endpoint":"http://leader.mesos/acs/api/v1/auth/login","private_key":"-----BEGIN PRIVATE KEY----\n<KEY>\n---END PRIVATE KEY----\n","scheme":"RS256","uid":"jenkins-principal"}',