Change value of None in encrypted volume field from "Maybe" to "-"

This makes it clearer to the user that information about whether the volume is
encrypted or not is not known.

Implemented by moving from standard yesno filter to filtering the value of the
encrypted field in the function get_encrypted_value

Change-Id: I14d157752e565fce68248af6c53a586ccf6c39be
Closes-Bug: 1354583