knowledge: API migration to SQLAlchemy

* Replaces run_sql queries with SQLAchemy query.

* Refactors module in order to move logic from api to model.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
`KnwKB.query.filter_by(name=kb_name).exists()` return a `sqlalchemy.sql.selectable.Exists` object