Check volume uniqueness by content, not name

[Ark-kun]

Related to #494 (comment)
Volumes should be deduplicated based on the volume itself, not the name. Then, if we have name conflicts, we should change the volume.name+volumeMount.name (we have to do that when processing a task (ContainerOp), because later we won't know which volume is used by which volumeMount).

One solution to this is to have a method like task.add_mounted_volume(volume, volume_mount) which will automatically set the volume name to volume structure hash. That way different volumes will get different names and same volumes will get same names.

[Ark-kun]

The issue was resolved by #2229