Changing the default for "Enter a passphrase to unlock the volume"

[Adam Funk]

(Background: I'm now using Ubuntu-MATE 18.04, but I've been wondering
about this for a while.)

When you insert an encrypted USB stick (e.g., LUKS), the "Enter a
passphrase to unlock the volume" dialog comes up, with three options:

  O Forget password immediately
  * Remember password until you logout
  O Remember forever

with the middle one as the default.  I want the first one (forget
immediately) to be the default --- is there any way to change that,
either per-user or for the system?

Thanks,
Adam