Mattermost-server: Add option for custom DB root/user password when using the mysql-operator for Mattermost operator

[jwilander]

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Notes: Jira ticket

The Mattermost Operator is a Kubernetes operator that manages the lifecycle of a Mattermost installation.

Currently, the root/user password for the database are randomly generated. However, some users might want to define a custom password.

For this ticket implement adding a field to the ClusterInstallation custom resource for the user to set a custom password for the root/user for the database.

Note that if the an external database is being used, this custom password field should be ignored.

[phillipahereza]

@jwilander I'll take this one

[Sheshagiri]

I will give this a try.

[jwilander]

https://github.com/mattermost/mattermost-operator/pull/80 should cover this