Legend labels in plot_ppc don't update to match 'prior' or 'posterior' group selection

[jedludlow]

Describe the bug
When calling plot_ppc using the matplotlib backend, the legend labels for the various data series in the plot do not update to reflect prior or posterior group. In many cases they are hard-coded to "Posterior ...".

To Reproduce
az.plot_ppc(prior_pred, kind='scatter', mean=True, group='prior'). The legend entries will still reflect "Posterior..." even though prior was selected.

Expected behavior
Legend entries should track the selected group.

Additional context
ArviZ v0.11.4 at a67d81c. I will submit a pull request with a solution.