Make `items` optional in `fromConfigMap()`

[nikolay]

In Kubernetes, you don't have to specify items and then all keys are added, but fromConfigMap() does not allow this and you end up having to specify each key, which is redundant, and hard to maintain.

[adamdecaf]

FYI, you can work around this by doing.

volume.withName(name) + volume.mixin.configMap.withName(name)