Adds support for tuning SQL Idle Timeout

Previously, Quantum logged frequent database-has-gone-away
errors with certain database configurations.

This patch adds a parameter that can configure
the sql_idle_timeout which can be decreased
to reduce these errors.

Defaults to '3600' to maintain backwards compatability.

Change-Id: I4ec0298c76bbf06aef79fe63fb5214afd4a2a703
Merge "Adds support for tuning SQL Idle Timeout"