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