chore(plugin/resouces): Get plugin resources by using ClassLoader

- get plugin resources as stream by using the ClassLoader of the
  CockpitPlugin SPI implementation
- at first the mechanism try to load the resources from classpath, if the
  result is equal null then it will load the resources from the webapp, so that the prefix "webapp://" is unnecessary.
- adjust test cases

related to CAM-1420
Merge branch 'master' of github.com:camunda/camunda-bpm-platform