Renames sql_dbpool_enable to dbapi_use_tpool

This option was renamed in
https://review.openstack.org/#/c/22163/
this patch fixes up the changes.

fixes bug 1087054

Change-Id: Ia172eeef08bb8405aa9e0598e793c4d4d218501d
Merge "Renames sql_dbpool_enable to dbapi_use_tpool"