When the internal VIP is moved in the event of a failure of the active
controller, OpenStack services can become unresponsive as they try to
talk with MariaDB using connections from the SQLAlchemy pool.

It has been argued that OpenStack doesn't really need to use connection
pooling with MariaDB [1]. This commit reduces the use of connection
pooling via two configuration options:

- max_pool_size is set to 1 to allow only a single connection in the
  pool (it is not possible to disable connection pooling entirely via
  oslo.db, and max_pool_size = 0 means unlimited pool size)
- lower connection_recycle_time from the default of one hour to 10
  seconds, which means the single connection in the pool will be
  recreated regularly

These settings have shown better reactivity of the system in the event
of a failover.

[1] http://lists.openstack.org/pipermail/openstack-dev/2015-April/061808.html

Change-Id: Ib6a62d4428db9b95569314084090472870417f3d
Closes-Bug: #1896635