Rework the "repo.bootstrap.ignore.content.append.conflict" functionality

In the past, we noted that during the upgrade, while bootstrapping with no baseline, there may be content nodes that already exist in the repository, and for which there is a content definition which is not yet marked as "applied". Under normal circumstances, we throw an ItemExistsException in this case, and the bootstrapping of this content definition will be skipped. During the upgrade, this is a to-be-expected situation, and to suppress the exception, we previously introduced a system property flag, to be raised during the upgrade.

In HCM-158, we found out that we should use a different approach, because BRC currently doesn't support setting system properties during deployments.

As an alternative (to be validated), we could introduce a temporary root-level node in the repository, which indicates that the upgrade of the repository from CMS 11 to CMS 12 has started. This node could be created automatically, by the repository. While the node exists, above-mentioned ItemExistsExceptions would be suppressed. Once the flag is no longer required, we would document a "clean-up" step in the upgrade documentation to go ahead and delete that node. The node would need to be categorised as 'system', which it is by default if it is a root-level node not defined in the ConfigurationModel.