Add description to explain cancellation of blockdev-wipe

Mostly as discussed in bug 722898. However, as it has turned out that the
different kinds of erase (non-crypto-volumes are overwritten with zeroes,
crypto-volumes are overwritten with random data) require different messages,
separate templates are used now.

The debconf progress bar frame doesn't print extended descriptions (at least
not in newt), therefore the message is squeezed into the short description
slightly awkwardly.
Use configured crypto options for volume wiping

Wiping of crypto volumes is done by setting up dmcrypt for the volume, filling
it with zeroes and then forgetting the key.  Previously, for the transient
crypto-volume, always the configuration "aes xts-plain64 128" had been used.
In the case the user chose a stronger setting, eg. "aes xts-plain64 256", this
meant that wiping occured with less security than encryption, i.e. metadata
was protected less than the actual data.

This changeset causes crypto_wipe_device() to read the encryption settings of
the to-be-wiped volume and to use them for wiping, too.
Add a bit of logging to crypto_wipe_device()
Clean up local variables in crypto_setup()