Abstract mysql endpoint/settings for unmanaged host

In order to allow people to use an external, already existing mysql server for
their OpenStack deployment, we need to be able to override the default role
based searches with manual attributes.

- Add get_mysql_settings which mirrors the default get_settings_by_role
 for mysql-master/mysql lookups

- Add get_mysql_endpoint which mirrors the default get_access_endpoint
 for mysql-server/mysql/db lookups

- Allow the override of the new methods using ["unmanaged"]["mysql"]
 attributes for host/server_root_password

- Change create_db_and_user to use the new *_mysql_* methods

- Add tests for attribute overrides for all of the above

See rcbops/chef-cookbooks#597