PMM2 Implement --create-user flag

User story:

UI/UX:

Acceptance criteria PO

Out of scope: P

Suggested implementation: Team on grooming/pre-grooming**

How to test: Team on grooming

Details:

as per PMM2 documention 

https://www.percona.com/doc/percona-monitoring-and-management/2.x/concepts/services-mysql.html#pmm-conf-mysql-user-account-creating

When adding a MySQL instance to monitoring, you can specify the MySQL server superuser account credentials. However, monitoring with the superuser account is not secure. If you also specify the --create-user option, it will create a user with only the necessary privileges for collecting data.

But for PMM2  pmm-admin flag '--create-user' is unknown.

# pmm-admin add mysql ps5726 127.0.0.1:5726 --username=msandbox --password=msandbox --query-source=slowlog --create-user
pmm-admin: error: unknown long flag '--create-user', try --help

Workaround: 

Create pmm user manually.

CREATE USER pmm@localhost IDENTIFIED BY 'pass' WITH MAX_USER_CONNECTIONS 10;
GRANT SELECT, PROCESS, SUPER, REPLICATION CLIENT, RELOAD ON *.* TO 'pmm'@'localhost';
GRANT SELECT, UPDATE, DELETE, DROP ON performance_schema.* TO 'pmm'@'localhost';

Add the instance as follows using pmm user,

sudo pmm-admin add mysql --user pmm --password pass