Explicitly set EDITOR=vi (unset on openSUSE / SLES)

Primarily this fixes annoying (log) messages from /usr/bin/quantum (due
to cliff/cmd2 usage):

which: no gedit in
(/sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/games:/usr/lib/mit/bin:/usr/lib/mit/sbin)
which: no kate in
(/sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/games:/usr/lib/mit/bin:/usr/lib/mit/sbin)

Furthermore, this simplifies introspection a bit since those commands
depend on $EDITOR:

    knife $THING edit $NAME
    crowbar $AREA proposal edit $PROPOSAL