Fix and refactor checks in check_interface_counters().
Fix and refactor checks in check_interface_counters().

- Added checks to ensure that interfaceCounters exist in both datasets at
  beginning of comparison.
- Refactored check in check_interface_counters to be more readable
- Fixed a bad check in check_interface_counters which skips counter checks
  unless we're interested in that counter. (config file)
- check_interface_counters now return does not contain anything for an interface
  when there are no differences.

arista-eosext/rphm