Fix bug when missing columns parameter for dashboard config.

Now you can provide any of the following combinations:

* Both "columns" and "width" values:
  Graphs will be displayed at the given width, and dashboard will show as many
  columns as specified of that given width, using overlay if necessary.

* Only "width", without columns:
  Graphs will be displayed at the given width and window will fit as many of
  them as possible.

* Only "columns", without width:
  Graphs width will be calculated by dividing the available space over the
  specified number of columns before displaying (not responsive!).
  Additionally, if "height" is not specified, it will be automatically
  calculated in a 16:9 ratio to the width