Specify image version

[ST-DDT]

Currently I cannot find an option to specify the mongodb-image path/version in the configuration.

I need this feature for:

• Precise controll on when to update and to which version
• Downloading from private/cached repositories

[pst]

This is a perfectly valid request. But since the operator is very far from being ready to run anything even remotely important with it I'll keep this open as a TODO for later.