inveniocfg: fix --drop-tables command

• Solves problems with droping tables using SQLAlchemy models. (closes #1283)

• Test note: may sometimes lead to "metadata lock" troubles, so beware. This will be clearly fixed in an ulterior commit.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>