Code review comment for ~mfo/ubiquity:automatic_prepare

Patch tested on both GTK and KDE frontends of ubiquity
with the Groovy Desktop ISO (daily-build of 2020-09-30),
now with proper, patched ubiquity packages.

Test Summary: (details in the next comments.)

If the user does not specify 'ubiquity/prepare_automatic',
the current behavior is maintained; ie, the prepare page
is _not_ included if in automatic mode.

If the user does specify it, only then the prepare page
is included in automatic mode.

If the user does specify it _and_ 'ubiquity/use_nonfree',
the prepare page is included and does not ask questions
(ie, consistent with other pages' behavior.)

The installer on GTK/KDE frontends finishes successfully.
The installer log copy shows the prepare page ran OK.

The ubiquity package built successfully on Groovy/Focal
for all architectures on http://pad.lv/ppa/mfo/lp1895351