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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
New Percona/MySQL 8 privileges with underscore is not working. 
the following error is coming 

`MSG: invalid privileges string: Invalid privileges specified: frozenset({'BACKUP_ADMIN'})`

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
mysql_user module
most likely the new privileges needs to be added to lib/ansible/modules/database/mysql/mysql_user.py

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

<!--- HINT: You can also paste gist.github.com links for larger files -->

module = AnsibleModule(
        argument_spec=dict(
            login_user=dict(type='str'),
            login_password=dict(type='str', no_log=True),
            login_host=dict(type='str', default='localhost'),
            login_port=dict(type='int', default=3306),
            login_unix_socket=dict(type='str'),
            user=dict(type='str', required=True, aliases=['name']),
            password=dict(type='str', no_log=True),
            encrypted=dict(type='bool', default=False),
            host=dict(type='str', default='localhost'),
            host_all=dict(type="bool", default=False),
            state=dict(type='str', default='present', choices=['absent', 'present']),
            priv=dict(type='raw'),
            append_privs=dict(type='bool', default=False),
            check_implicit_admin=dict(type='bool', default=False),
            update_password=dict(type='str', default='always', choices=['always', 'on_create']),
            connect_timeout=dict(type='int', default=30),
            config_file=dict(type='path', default='~/.my.cnf'),
            sql_log_bin=dict(type='bool', default=True),
            client_cert=dict(type='path', aliases=['ssl_cert']),
            client_key=dict(type='path', aliases=['ssl_key']),
            ca_cert=dict(type='path', aliases=['ssl_ca']),
            plugin=dict(default=None, type='str'),
            plugin_hash_string=dict(default=None, type='str'),
            plugin_auth_string=dict(default=None, type='str'),
        ),