Create unit test for VMscreenshot.pm
* Extend test data
* Mock HTTP service with local file
* Adjust other tests for changed test data
Various little fixes and improvements
* Add test png file
* pin push_max to 25 in test config
* Retrieve only required properties vom vSphere in get_vm_data in an
  attempt to boost performance (did not help :-())
* Set special header for last image in LML::VMscreenshot::render
Fix uninitialized value error in vmdata.pl