autoinstall: comment out/empty partman_early_command

Currently only used for a karmic-specific workaround.

Change-Id: Idc02ddd2560314cc5cea488eca9989a53063dcf9
autoinstall: setup network-console within d-i

network-console sets up an SSH server within the installer, which can be
useful for debugging, especially when it comes to e.g. partman recipes.

Set it up and use public key authorization, using our preexisting
new_install key.

Change-Id: I0d25bd66098c7c2976d09fc6c6da6bf2a2110c6c