Init containers image defaulting

[thbkrkr]

This commit makes sure that user provided init containers inherit from the default values including the main container image.

I refactored a bit the functions that build the PodTemplateSpec to try to make them boring and always have the same pattern by preparing the volumes, volumeMounts, initContainers and then only call:

		WithVolumes(volumes...).
		WithVolumeMounts(volumeMounts...).
		WithInitContainers(initContainers...).
		WithInitContainerDefaults()

Resolves #3453.

[thbkrkr]

jenkins test this please

[sebgl]

LGTM