Swallow rm_na arg in all H2OFrame methods in Python and use skipna instead
A while back, we changed the `rm_na` arg to `skipna` to be compatible with Pandas. This was probably done to several functions, but for sure this is the case on the `h2o.H2OFrame.mean` method. It was reported here: https://stackoverflow.com/questions/44910513/h2o-apply-python-function-seems-to-be-broken
Unfortunately for the mean method, it was not made backwards compatible at the time of the change. For all the frame functions, ideally we would support both arguments, with `skipna` being the documented name.
The Python booklet also needs to be updated once this ticket is complete (and we need to make sure there are no old demos using rm_na as well).