Use a single change ID per requirement proposal.

* modules/jenkins/files/slave_scripts/propose_requirements_update.sh:
This script was appending Change IDs to the commit message each
iteration through the list of projects which creates a mess in Gerrit.
Reset the commit message at the start of each loop iteration to ensure
each change has a single Change ID.

Change-Id: I6be2ce752c784f0449f90b28a04d4b13a653256e
Merge "Use a single change ID per requirement proposal."