Repaired the usage of ``.prepare()`` in conjunction with

Repaired the usage of ``.prepare()`` in conjunction with
cx_Oracle so that a return value of ``False`` will result
in no call to ``connection.commit()``, hence avoiding
"no transaction" errors.   Two-phase transactions have
now been shown to work in a rudimental fashion with
SQLAlchemy and cx_oracle, however are subject to caveats
observed with the driver; check the documentation
for details. [ticket:2611]