Use utf8_general_ci collation as a default collation

Now, I see mariadb are using utf8_general_ci as a default collation.
- https://mariadb.com/kb/en/mariadb/supported-character-sets-and-collations/

This mean all of Devstack database will be created with utf8_general_ci collation,
so may be, one service/project can be deployed successfully via Devstack
but will be fail with Kolla deployment.

Therefore, we should use above default collation for Kolla-ansible.

Change-Id: Icbb6c15f536fc6986816c58f4fd68bfb95813e46
Closes-Bug: 1680783