Run session creation in system context

In an effort to get rid of the periodic failing
tests with:
```
java.lang.IllegalArgumentException: No valid permissions available!
DUMMY permissions are not intended for copying. Make sure that you have
not passed omero.group=-1 for a save without context
```
make all calls to `SessionManagerImpl.createSession` run with an
explicit context for the system group.

/cc @ximenesuk

Note: this does not (yet) fix the problem of *why* the root session is being set to -1.