Add support for SQL idle timeout.

Previously, nova 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: I1b0e2c763b3e5c9d3310bfd36ec0f54189e4996d
Merge "Add support for SQL idle timeout."