Add support for SQL idle timeout.

Previously, OpenStack services logged
frequent database-has-gone-away errors.

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

More information on this issue can be found here:
  https://bugs.launchpad.net/nova/+bug/1007027 for more details.

Change-Id: I6e9388b36207be87f7e706be18b4cd4060956a55
Merge "Add support for SQL idle timeout."