Convert explicit session get/begin to transaction context

Convert the explicit common.sql.get_session() and subsequent
session.begin() to the transaction context manager for Trust
SQL backend.

Change-Id: I52745118782301ef26452063628e16e92e8ee276
Do not log 14+ INFO lines on a broken pipe error (eventlet)

Under eventlet if the client disconnects while Keystone is trying
to transmit data, approximately 14 lines of traceback are logged.

This could result in massive logs if done repeatedly; worst case
scenario could include a denial of service due to disk space being
exhausted.

Change-Id: Iaf14c64799daa7539d1bd85cb6c6457380c111d9
Closes-Bug: #1334466
Merge "Convert explicit session get/begin to transaction context"
Merge "Do not log 14+ INFO lines on a broken pipe error (eventlet)"