CLOUDIFY-2251

Verify plugin will raise an exception if the expected operation is not registered within the celery worker
CLOUDIFY-2251

Improve exception description to specify the registered plugin operations
CLOUDIFY-2251

1. Separate plugin installer verification method and get arguments into two methods.
2. verify method has a throw_on_failure indicator whether it should raise an exception or return False when verification is failed
Merge pull request #4 from CloudifySource/feature/CLOUDIFY-2251_transfer_installed_plugin_knowledge

split verify method into verify and get arguments, added mode in which the verify method will throw an exception and a mode in which it will return 'False'