8095: Make install guide bundle commands more consistent.

8095: Make install guide bundle commands more consistent.

* Always instruct the user to use `sudo -u`.  This prevents the
  commands from generating cache files that the web server can't write
  later.
* Always execute the command in the server's main directory.  This
  helps people run on automatic: if they leave a shell session open at
  the API server directory, they can paste in a combination of Rails
  console and script commands without `cd`ing around.
* Always give both RVM and non-RVM instructions.  There might be a
  good way to streamline this, e.g., by telling them to use rvmsudo
  when available, but that can be a later optimization.