Add vfiler support

NetApp allows multiple virtual filer to run on one physical filer. Every
API command runs under a specific vfiler context.

Managing a vfiler while connecting to the actual physical filer has the
benefit that you do not have to have network access to your vfiler
(e.g. you can place a vfiler in a DMZ network while your physical filer
is only accessable through your internal network).

The NetApp Server class already implements a method `set_vfiler` that
changes the context for all following api calls.

Allow passing a vfiler via the connection url. So instead of using

    [filer.example.com]
      type netapp
      url https://user:pw@filer.example.com

we accept the connection url `https://user:pw@filer.example.com/[virtalfiler]`