Documentation build fails
Motivation
----------
Currently the documentation builds fail with 

```
Running Sphinx v1.6.5
making output directory...
loading translations [en]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/rucio/envs/latest/local/lib/python2.7/site-packages/sphinx/cmdline.py", line 305, in main
    opts.warningiserror, opts.tags, opts.verbosity, opts.jobs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rucio/envs/latest/local/lib/python2.7/site-packages/sphinx/application.py", line 196, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rucio/envs/latest/local/lib/python2.7/site-packages/sphinx/application.py", line 456, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rucio/envs/latest/local/lib/python2.7/site-packages/sphinx/registry.py", line 199, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname, err)
ExtensionError: Could not import extension rucio.common.doc.argparse.ext (exception: No module named rucio.common.doc.argparse.ext)

Extension error:
Could not import extension rucio.common.doc.argparse.ext (exception: No module named rucio.common.doc.argparse.ext)
```

I did not look too much into the issue yet, but I wonder why we added this `rucio.common.doc.argparse.ext` to the repository instead of using `sphinx-argparse`? @vingar do you maybe remember?
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 60e19bdfa..99bdb9c1f 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -20,7 +20,7 @@ import os
 import sys
 from mock import Mock as MagicMock
 
-sys.path.insert(len(sys.path) - 1, os.path.abspath('.'))
+sys.path.insert(len(sys.path), os.path.abspath('../../lib'))
 
 
 class Mock(MagicMock):
['doc/source/conf.py']