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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 -->

current_plugin = cursor.fetchone()
            update = False
            if current_plugin[0] != plugin:
                update = True
            if plugin_hash_string and current_plugin[1] != plugin_hash_string:
                update = True
            if plugin_auth_string and current_plugin[1] != plugin_auth_string:
                
                
                
                
                update = True
            if update:
                if plugin_hash_string:
                    cursor.execute("ALTER USER %s@%s IDENTIFIED WITH %s AS %s", (user, host, plugin, plugin_hash_string))
                elif plugin_auth_string:
                    cursor.execute("ALTER USER %s@%s IDENTIFIED WITH %s BY %s", (user, host, plugin, plugin_auth_string))
                else:
                    cursor.execute("ALTER USER %s@%s IDENTIFIED WITH %s", (user, host, plugin))
                changed = True
        
        if new_priv is not None:
            curr_priv = privileges_get(cursor, user, host)
            
            
            for db_table, priv in iteritems(curr_priv):
                
                if "GRANT" in priv:
                    grant_option = True
                if db_table not in new_priv: