BUG: inconsistent subplot ax handling
There is an inconsistency `DataFrame.plot`, `hist` and `boxplot` when subplot enabled (`subplots=True` or use `by` kw).
### Current behaviour
- `plot` and `hist`:
  When `ax` kw is passed, plot will be drawn on the figure which the passed ax belongs. The figure will be once cleared even if the required number of subplot is 1. Thus, any artists contained in the passed `ax` will be flushed.
- `box`:
  When `ax` kw is passed and a required number of subplots is 1, use the passed `ax` without clearing the existing plot. If more than 1 axes is required for the subplots, raises `ValueError`.
### Fix
- When `ax` kw is passed and a required number of subplots is 1, use the passed `ax` without clearing the existing plot.
- If more than 1 axes is required for the subplots, subplots will be output on the figure which the passed ax belongs. The figure will be once cleared if the number of subplots is more than 1.
