use another way to not auto-indent in python mode

Tried out the suggestion here https://github.com/syl20bnr/spacemacs/pull/126, and it works fine. Let's go with this approach, so that people who likes `electric-indent-mode` can have it in python mode(although I can't think of any case so far).

I didn't remove `disable-electric-indent` function that I added in previous PR, let me know if you'd like not to have it anymore.

I also bind `C-j` the same way as `<RET>` key as I find myself use those two key bindings interchangeably, but that's totally something subjective, so feel free to let me know if you don't like it and I can take it out.
We can set it globally in `spacemacs/keybindings.el`