scylla_node: add a helper for accessing environ variables

[tchaikov]

scylla_node: add a helper for accessing environ variables

before this change, we always try to access self._launch_env and fall back to os.environ, and optionally update the return env dict, then use it. the pattern repeats itself couple times in ScyllaNode class.

in this change, to consolidate it, a helper is added. it does not do anything clever just preserve existing behavior. despite that existing behavior could be potentially confusing: we actually mutate the returned dict if it references the existing self._launch_env.

[tchaikov]

v2:

• instead of initializing self._launch_env in constructor, introduce a helper for accessing / updating it.

@fruch hi Israel, could you help re-review this change?