Clean up traffic_profile vs osi_layer

The two types were not well distinguished before.

Error introduced in:
https://gerrit.fd.io/r/#/c/17811/84/resources/libraries/robot/performance/performance_setup.robot@255

Error hotfixed in:
https://gerrit.fd.io/r/#/c/18847/4/resources/libraries/robot/shared/default.robot@109

+ Rename some arguments and improve method docstrings.

Newly introduced argument name osi_layer should be dissimilar enough.

Change-Id: Ie0f6f97dc010fc6477f09c54574970f1d15462e2
Signed-off-by: Vratko Polak <vrpolak@cisco.com>