Bug#396023: Allow encrypted partitions to be removed
The issues about re-installing with partman-auto-lvm on top of stale
lvm metadata on disk is solved with a patch in 
My take on solving "Should allow de-allocation of encrypted volumes"
bug 381892 is actually resolving the TODO item in partman-crypto:
"Understand and add undo.d scripts if required"
My understanding is that all cryptsetup dm-devices should be closed.
That implies deactivating any dm volumes that sit on top of cryptsetup
device (e.g. lvm) as well as unlocking the underlying physical
I am not sure if this should be done before or after 30partman - i.e.
parted UNDO command.
Naturally this will also imply that patch from bug 451535  should
be applied, which allows activating existing cryptsetup volumes.