Call self._get_tools() once in a life time of an object

[aglyzov]

Too make it more efficient we should only call self._get_tools() once, presumably inside init(). This is especially true for Green classes since they use expensive utils.detect_green_env() and utils.get_green_tools() under the hood.

[AxelVoitier]

Indeed, I was not aware it was an expensive call. I will try to fix that.