In some situations, especially with older versions of clevis, we can end
up with a partially used luksmeta slot.
We can identify such slots because they will be marked as inactive, yet
they will contain the clevis UUID, "cb6e8904-81ff-40da-a84a-07ab9ab5715e".
When this situation happens, we have cryptsetup and luksmeta slots "out
of sync", and since we currently have cryptsetup choose the slot, we may
end up trying to use such a partially used slot, which in turn will fail
because luksmeta will not be able to save data to it.
We handle this case by wiping the partially used slot, if we identify
the situation will arise.
Tests also added to verify this case is handled properly.
Fixes: #70
