[mysql_metrics] Replace Global Master Password

[BugRoger]

https://github.com/sapcc/helm-charts/tree/master/common/mysql_metrics

• Remove all references to Values.global.master_password
• Prefer to not reference a global value. Instead require a password being passed to the chart

Prior Art:
a4c48f4

[galkindmitrii]

We don't use master_password for mariadb of mysql metrics.

[BugRoger]

We don't use master_password for mariadb of mysql metrics.

Perfect. I suppose, the reference here can be deleted then:
https://github.com/sapcc/helm-charts/blob/master/common/mysql_metrics/templates/_helpers.tpl#L24

[galkindmitrii]

Ah, that can be the case here: bf5570d

[Carthaca]

It is fair that this task is assigned to me - I'm a heavy user of the master password algorithm ^^