sortMounts stable and remove alphabetical order

[Epsilon314]

PR for #240

[elezar]

Thanks @Epsilon314

What is the expected behaviour when the mount being injected has:

1. the same number of parts
2. a greater number of parts

The behaviour for 1 seems undefined and was the motivation for the alphabetical sorting in the first place.

[Epsilon314]

@elezar For mounts with same number of parts, its original order will be kept (stable sort). Since containerd also uses stable sort, users can therefore arrange the order of their mounts (of same number of parts ) in container spec if they do need a specific order.

[elezar]

Ok, so if we add mounts these would always be added after the existing mounts. Could you add tests for those cases?

[Epsilon314]

Ok, so if we add mounts these would always be added after the existing mounts. Could you add tests for those cases?

@elezar OK

[elezar]

Thanks @Epsilon314.

@klihub @bart0sh please take a look.