start elpy-rpc process with the current buffer's process-environment

If a user made process-environment buffer local and added environment
variables to support his use-case, those changes would be lost to the
elpy-rpc process if it was started in the context of a new buffer.

Possible scenario: A dedicated virtualenv for all emacs related dev
packages (elpy, PyLint, ...) which is added via PYTHONPATH to each
opened python buffer. In that buffer, exec-path and process-environment
are buffer local to isolate those changes and thus make it possible to
work on projects with different Python versions and/or virtualenvs.