Change db_uri to hand out UTF8 MySQL URIs

MySQL databases normally default to use whatever
character set is read from the global config.
OpenStack projects expect to be working with a unicode
charset database, so lets make sure that assumption is
true by explicitly stating UTF8 in the connection string.

Closes-Bug: 1257056

Change-Id: Iac63b40f603d1144ae3c6575b372ad586dabd4b9