Move the passphrase and iteration count into variables.

This is purely to make it easier to tweak them locally; the machine I have
for testing takes forever to do 50,000 pw strengthening iterations, and
we're not testing the strength of geli's anti-pw-guessing logic here
(especially given that our test passphrase is "passphrase", except that
I tend to tweak that also, to 'x', because typing is hard).

Some day these should be settable as cmdline args. But then, some day this
whole script should probably get a rewrite. :)