Add support for SQL idle timeout.

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

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

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

Also cleans-up inline Puppet documentation.

Change-Id: I1479844a88ab9047e78c5060231c500f38b38720
Merge "Add support for SQL idle timeout."