Fix error in two-step secret PUT with base64

An attempt to PUT base64-encoded binary data as the 2nd step in storing
a secret resulted in a SQLAlchemy 'already attached in another session'
error with the secret model. This CR moves to the SQLAlchemy scoped
session which ensures one session per thread. This CR also adds a
simple 'smoke test' script that was used to exercise the API during the
development of this CR, and could be helpful for others looking for a
simple way to test and evaluate their local Barbican API instances.

Closes-Bug: #1374270
Change-Id: Ie50e5f03be7b9b12d82a285a71e2f1b5c2483ea0