Allow for process initiator to be set using 'initiator' value from process data

When setting Bypass User Authentication, everytime a new process is started, the new process is audited as being started by the connection user and can't be set other.

This enhancement allows for the audited user to become from process data, at the time the process is started, using the variable 'initiator'.

The behaviour would be, if bypass user auth is set and when starting a process, the process data contains a value for 'initiator', that value would be used, otherwise the value from the identityProvider.

Community issue:
https://groups.google.com/forum/#!topic/jbpm-usage/Z8u82iERFRE