InvenioUpgrader: partial fix for SQLAlchemy init

• InvenioUpgrader imports pluginutils which imports webinterface_handler which in the end results in an SQLAlchemy model being imported. The model then raises an exception due to the SQLAlchemy engine not being initialized. This partial fix ensures the engine is initialized by loading invenio.flaskshell. However, 'make check-upgrade' will not work for non-SQLAlchemy based installations, but will nonetheless report this.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>