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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 -->

""" Take a privileges string, typically passed as a parameter, and unserialize
    it into a dictionary, the same format as privileges_get() above. We have this
    custom format to avoid using YAML/JSON strings inside YAML playbooks. Example
    of a privileges string:
     mydb.*:INSERT,UPDATE/anotherdb.*:SELECT/yetanother.*:ALL
    The privilege USAGE stands for no privileges, so we add that in on *.* if it's
    not specified in the string, as MySQL will always provide this by default.
    """
    if mode == 'ANSI':
        quote = '"'
    else:
        quote = '`'
    output = {}
    privs = []
    for item in priv.strip().split('/'):
        pieces = item.strip().rsplit(':', 1)
        dbpriv = pieces[0].rsplit(".", 1)
        
        parts = dbpriv[0].split(" ", 1)
        object_type = ''
        if len(parts) > 1 and (parts[0] == 'FUNCTION' or parts[0] == 'PROCEDURE'):
            object_type = parts[0] + ' '
            dbpriv[0] = parts[1]
        
        
        for i, side in enumerate(dbpriv):
            if side.strip('`') != '*':
                dbpriv[i] = '%s%s%s' % (quote, side.strip('`'), quote)
        pieces[0] = object_type + '.'.join(dbpriv)
        if '(' in pieces[1]: