Make DTL assignment independent of vPool

[kvanhijf]

Currently the StorageRouter to which the DTL points for a vDisk, is always selected within the vPool to which the vDisk belongs. According to @redlicha this is not a requirement.

you can point a volume to a DTL wherever you feel like - someone (you :)) has to make sure it's running there and that's it voldrv doesn't care

We are maintaining this principle in the framework, because of the chance that 1 cluster can be used for several customers each owning their vPool in a specific vLAN.

PS: This ticket was created solely to remember we have this option, but currently we don't make use of it.