rest, indexer: store creators of the resources

This patch adds storing the user/project who created the resource. This
is going to be needed in order to allow finer-grained rules in the
policy. It will for example disallow create/update/delete operations on
resources/entities that were created by an admin on behalf of a user.

Change-Id: I6916e0d5da2115ea496141e69ceb39d861fa92ae
Merge "rest, indexer: store creators of the resources"