<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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 -->

if host_all:
        cursor.execute("SELECT count(*) FROM mysql.user WHERE user = %s", ([user]))
    else:
        cursor.execute("SELECT count(*) FROM mysql.user WHERE user = %s AND host = %s", (user, host))
    count = cursor.fetchone()
    return count[0] > 0
def user_add(cursor, user, host, host_all, password, encrypted,
             plugin, plugin_hash_string, plugin_auth_string, new_priv, check_mode):
    
    if host_all:
        return False
    if check_mode:
        return True
    if password and encrypted:
        cursor.execute("CREATE USER %s@%s IDENTIFIED BY PASSWORD %s", (user, host, password))
    elif password and not encrypted:
        cursor.execute("CREATE USER %s@%s IDENTIFIED BY %s", (user, host, password))
    elif plugin and plugin_hash_string:
        cursor.execute("CREATE USER %s@%s IDENTIFIED WITH %s AS %s", (user, host, plugin, plugin_hash_string))
    elif plugin and plugin_auth_string:
        cursor.execute("CREATE USER %s@%s IDENTIFIED WITH %s BY %s", (user, host, plugin, plugin_auth_string))
    elif plugin:
        cursor.execute("CREATE USER %s@%s IDENTIFIED WITH %s", (user, host, plugin))
    else:
        cursor.execute("CREATE USER %s@%s", (user, host))
    if new_priv is not None:
        for db_table, priv in iteritems(new_priv):
            privileges_grant(cursor, user, host, db_table, priv)
    return True
def is_hash(password):