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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 -->

mysql_user:
    state: present
    name: bob
    password: 12345dd
    priv:
      'db1.*': 'ALL,GRANT'
      'db2.*': 'ALL,GRANT'
# Note that REQUIRESSL is a special privilege that should only apply to *.* by itself.
- name: Modify user to require SSL connections.
  mysql_user:
    name: bob
    append_privs: yes
    priv: '*.*:REQUIRESSL'
    state: present
- name: Ensure no user named 'sally'@'localhost' exists, also passing in the auth credentials.
  mysql_user:
    login_user: root
    login_password: 123456
    name: sally
    state: absent
- name: Ensure no user named 'sally' exists at all
  mysql_user:
    name: sally
    host_all: yes
    state: absent
- name: Specify grants composed of more than one word
  mysql_user:
    name: replication
    password: 12345
    priv: "*.*:REPLICATION CLIENT"