Adds Support for Glance Registry MySQL Idle Timeout

Previously, only the glance::api class could manage the sql idle
timeout.  This change allows the sql idle timeout for Glance
Registry to be configurable.

Defaults to '3600' for backwards compatibility with a
`wait_timeout' of 8 hours, after which MySQL will drop idle
connections.

Change-Id: I00d01e77af56384f4d74ab8de4a7d3bbd298e9a2
Merge "Adds Support for Glance Registry MySQL Idle Timeout"