Fix mysql_user on_new_username IndexError (#642)

* fix tuple indexerror when no accounts are found

* Fix tests for update_password not executed

* Add test for case where existing user have different password

* lint to prevent warning about jinja templating in when clause

* Refactor get_existing_authentication to return a list of all row found

Previously we were returning only the first row found. We need to be
able to see if there is a difference in the existing passwords.

* Refactor host option to be optional

This make it possible to use the same method from mysql_user to help
update_password retrieve existing password for all account with the same
username independently of their hostname. And from mysql_info to get
the password of a specif user using WHERE user = '' AND host = ''

* Add change log fragment

* Add link to the PR in the change log

* lint for ansible devel

* Fix templating type error could not cconvert to bool with ansible devel

* Revert changes made for ansible-devel that broke tests for Ansible 2.15

* Revert changes made for ansible-devel that broke tests

* Cut unnecessary set, uniqueness is ensured by the group_by in the query

* Cut auth plugin from returned values when multiple existing auths exists

Discussed here:
https://github.com/ansible-collections/community.mysql/pull/642/files#r1649720519

* fix convertion of list(dict) to list(tuple)

* Fix test for empty password on MySQL 8+

plugins/modules/mysql_info.py

@@ -639,7 +639,7 @@ class MySQL_Info(object):
 
             authentications = get_existing_authentication(self.cursor, user, host)
             if authentications:
-                output_dict.update(authentications)
+                output_dict.update(authentications[0])
 
             # TODO password_option
             # TODO lock_option