Add support for SQL idle timeout.

Previously, keystone logged frequent
database-has-gone-away errors.

This patch adds a parameter that can configure
the 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: I1731d6ef1138e82965148fdfc1fe07301c873d47
Merge "Add support for SQL idle timeout."