support --random-order-bucket=no (or =none)

[anntzer]

Supporting --random-order-bucket=no (or none, I don't really care) would allow that option to be listed in the output of pytest -h, avoiding the need for the occasional user to lookup online the syntax to disable a plugin (-p no:random-order).

[jbasko]

Thanks for the suggestion. Done in v0.5.6 with none