Fix change in 70cb792c7 to show extra_source
Fix change in 70cb792c7 to show extra_source

Changing the isinstance check for python3 compatibility broke the
display of extra data from the loaded YAML file.
Either unicode or str are suitable for os.path.exists

Change-Id: Iad0d093fb182047b466f6e487a6574b67754b7c7

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server