simplify --debian-installer choices

`false` and `none` make no sense as choices for this option. Here we
replace `false` with `none`, and remove `true`.

Note that `true` was treated as an alias for netinst (see the changes to
source_disk and and binary_disk).

For backwards compatibility we still allow `true` and `false` by converting
them to `netinst` and `none` respectively, whilst printing a warning to
encourage users to move to `netinst`/`none`.

Gbp-Dch: Short
Closes: #952864

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