Fixes authentication error when using oracle (#1193)
Fixes authentication error when using oracle (#1193)

This commit applies for oralce dialect the same query strategy already implemeted for MSSQL in
security manager when checking for authentication/permissions, since a
query like "SELECT EXISTS(1) FROM DUAL" in oracle databases would raise
an ORA-00936: missing expression error (observed from version 18c on)
dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder