PE-1620: handle class names in retry_exception hash co

Previously the +Puppet::CloudPack::Utils.retry_action+ method was able to handle ```retry_excpetion``` hash in which the exceptions were specified by strings (or symbols). But it failed when the ```--debug``` option was specified at the same time because it tried to lookup the caught exception description in the ```retry_exception```
hash using the exception class object as a key to the hash instead of the class name string (or symbol).

This was fixed so now the method can handle exceptions specified as string (or symbols) correctly under all circumstances.
A test was also added to verify the fix. And while at it, the affected spec file syntax was modernized.