* dict.iteritems does not exist in Python 3
Now just dict.items
six.iteritems handles the change
* Addresses point 1
Unsure if this is a good idea or not.
* Addresses point 2
This shouldn't have any particular change, just marks load_comments as abstract
* Remove unused import
Addresses point 3
* Clarify invalid subset error message
Addresses point 4
* adds support for std network facts
* adds support for default facts subset
* adds support for config facts subset
* adds support for interface facts subset
* adds support for hardware facts subset
Tested on EOS 4.15.4F
