Fix warning in lyx2lyx 2.0<->2.1 round trip

Fix warning in lyx2lyx 2.0<->2.1 round trip

revert_justification() issues a warning if the \justification parameter does
not exist, and LyX itself always writes it unconditionally as well, so add it
also in lyx2lyx when converting old documents.
Currently this does not have any user visible effect, but it decouples the
default value of BufferParams::justification from the conversion of old
documents: Now it is possible to set the default to false in LyX, and old
documents will still be converted correctly.