Closes #2003: Fix NXOS ping source_interface bugs

[decoupca]

Fixes two source_interface bugs in NXOS driver, whereby passing either an invalid or valid source_interface parameter returns an empty data structure.

[decoupca]

This PR solves my original issue, but it's a good idea to include an else statement after the block starting on line 379 to catch any future similar errors. Should that raise an exception or just add the output to ping_dict["error"] like the first if statement?

[mirceaulinic]

@decoupca yes, I think it should explicitly raise an exception. Thanks!

[mirceaulinic]

@decoupca is this still something you'd like to continue on?