[MRG + 1] 18 more examples with matplotlib 2.0 updates (#8983)

* updated plot_label_propagation_versus_svm_iris.py plot

* updated svm/plot_weighted_samples.py plot

* made semi_supervised/plot_label_propagation_versus_svm_iris.py pep8 compliant

* modified tree/plot_tree_regression.py [size and edgecolor]

* updated tree/plot_tree_regression_multioutput.py [size+color]

* fixed examples/semi_supervised/plot_label_propagation_versus_svm_iris.py for backward compatibility

* neural_networks/plot_mlp_alpha.py - matplotlib2 update

* examples/neural_networks/plot_mlp_alpha.py - pep8 fix

* examples/neighbors/plot_nearest_centroid.py - matplotlib2.0 + pep8 fix

* neighbors/plot_classification.py - matplotlib2.0 + pep8 fix

* examples/neighbors/plot_lof.py - matplotlib2.0 update

* examples/model_selection/plot_underfitting_overfitting.py - matplotlib2.0 + pep8

* examples/mixture/plot_concentration_prior.py - matplotlib2.0 + pep8

* examples/linear_model/plot_logistic_multinomial.py - matplotlib2.0 update

* linear_model/plot_sgd_iris.py - matplotlib2.0 + pep8 fix

* examples/linear_model/plot_sgd_weighted_samples.py - matplotlib2.0 + pep8

* examples/linear_model/plot_sgd_separating_hyperplane.py - matplotlib2.0 update

* examples/feature_selection/plot_permutation_test_for_classification.py - matplotlib + pe8

* examples/linear_model/plot_bayesian_ridge.py - matplotlib2.0 update

* examples/feature_selection/plot_feature_selection.py - matplotlib2.0 update

* examples/feature_selection/plot_f_test_vs_mi.py - matplotlib2.0 + pep8

* examples/feature_selection/plot_f_test_vs_mi.py - matplotlib2.0+ pep8 fix

* examples/model_selection/plot_underfitting_overfitting.py - error fixed

* blue -> black edgecolor fix for 2 examples