Add a merge step to our Jenkins ISO build process

In our automatic builds specs, we designed that we wanted the build of feature branches to be done after it has been merged locally in their base branch.

We still need to implement this, as at the moment we only build from the feature branch.

This will probably mean modifying our vagrant/provision/assets/build-tails script.

Parent Task: #6196 (closed)

Original created by @bertagaz on 9601 (Redmine)