code: public static String getPluginConfiguration(MavenProject proj, String pluginGroupId, String pluginArtifactId, String key) {
          Xpp3Dom dom = proj.getGoalConfiguration(pluginGroupId, pluginArtifactId, null, null);
          if (dom != null) {
              Xpp3Dom val = dom.getChild(key);
              if (val != null) {
                  return val.getValue();
              }
          }
          return null;
  }
docs: Get a configuration value from a plugin
  @param proj
  @param pluginGroupId
  @param pluginArtifactId
  @param key
  @return