Deprecate min_pool_size
Deprecate min_pool_size

This option has for a long time done nothing in oslo.db
and sqlalchemy does not support this convention. As such I
recommend that we deprecate and eventually remove the option.

Closes-bug: #1764786

Change-Id: Ib07b7b4200b5cf3a34b8eed441e6754da7d446cf

openstack/oslo.db,openstack/oslo.db