support simple headless install (without either encryption or resize)

[Tom Roche]

Ideally, a user of this project could use this code merely for a headless but otherwise "normal" LMDE install. (This might also be useful for testing.) Presumably install_LMDE_plus_LUKS_LVM2.properties could support appropriate flags, to be read/used by install_LMDE_plus_LUKS_LVM2.sh.