Add eventlet db_pool use for mysql

Add eventlet db_pool use for mysql

This adds the use of eventlet's db_pool module so that we can make mysql
calls without blocking the whole process.
New config options are introduced:

sql_dbpool_enable -- Enables the use of eventlet's db_pool
sql_min_pool_size -- Set the minimum number of SQL connections
sql_max_pool_size -- Set the maximum number of SQL connections
sql_idle_timeout  -- Timeout before idle sql connections are reaped

The default for sql_dbpool_enable is False for now, so there is
no forced behavior changes for those using mysql. sql_min_pool_size
is defaulted to 1 to match behavior if not using db_pool.

Fixes bug 1086173

Change-Id: Ied0aae33211585743fe955028a75c4e192a15d2f