fix #12160: root-owned template paths (rebased from dev_5_0)

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12160 to allow managed repository template paths to start with some root-owned directories.

In `etc/omero.properties` the `omero.fs.repo.path` now has a `//` for one of the path separators. Imports should still work fine, the images remaining usable, with those earlier directories before the `//` owned by root instead of by the user.

One can use `bin/omero hql` to check directory ownership with queries like,
```
SELECT path || name, details.owner.omeName, details.group.name FROM OriginalFile WHERE mimetype = 'Directory'
```

--rebased-from #2448