Use eventlet.tpool.Proxy for DB API calls

There is no eventlet in common db code anymore, so, if we want to use
thread pooling for DB API calls, we should use eventlet.tpool.Proxy()
instead of tpool_wrapper() from openstack.common.db.api.

Config option CONF.database.use_tpool was moved from module
nova.openstack.common.db.api to nova.db.api.
Config file `nova.conf.sample` was updated as well.

See also patch to Oslo - I6fa140f2b1f03c43ae7cab98ac8a12d71188af76

Co-Authored-By: Chris Behrens <cbehrens@codestud.com>
Change-Id: Id7de85dce11f5ea61dfccddf80de7800a2bc4e37
Merge "Use eventlet.tpool.Proxy for DB API calls"