Set utf8 encoding for mysql and postgresql

Currently, it's enforced in devstack through setting encoding in
connection URI. It would be beneficial if oslo.db handles that on its
own, so that we can

- remove those connection arguments from devstack;
- more importantly, not rely on users to set it in all affected
  projects.

Oursql and MySQLdb also require use_unicode=0 to avoid performance drop.

Change-Id: I676c9c5e418905160e4ab647b8a0f2fb601e8962
Closes-Bug: 1340779