Bug#451535: debian-installer: allow to 'reuse' encrypted volumes

[draeath]

There's another use case here too, in cases where I want to use my own LUKS
parameters - that the installer doesn't expose. Perhaps I want
--type=luks1, maybe I want a different --iter-time, etc. Actually, if the
installer (in expert mode) let us pass arbitrary arguments for cryptsetup
this would close up this use case.