pmm-admin add mysql CLI ignoring username parameter

We started having an issue with pmm-admin add mysql command, where when a user would provide --username parameter, CLI would completely ignore this and use default root. 

This is causing many tests to fail on our jobs

STR: 

1) setup mysql Database, add a non root user, example pmm user

2) Use PMM user to connect to pmm-server for monitoring this database

3) CLI would try to use the default username "root" even when you pass --username="pmm"

User Impact:

Adding mysql DB for monitoring will not work, because username is being ignored.