Move volume manager specific urls into extension
Move volume manager specific urls into extension

* added a field for extension where urls can be
  defined
* move urls into volume manager extension
* removed TODO from test_api_doc_generation test
* removed TestRemovedIn51Handler test because:
** it does not test anything, correct work
   of removal of related handler is tested in
   in other tests in the same file
** the test relies on patching of global variable
   which causes a lot of problems, now you cannot
   change global variable so it gets propagated
   into web.py
* added exception H304 in tox configuration, it's
  required because extensions should be written in
  a way that they can import their modules without
  specifing full path to extension, which currently
  includes nailgun module as a root module. It will
  be easier to extract it later from nailgun into
  separate module/repo

Implements blueprint: volume-manager-refactoring

Change-Id: I6fd5a63e6946ef87604f1f91a99351c24de45689

gitfred/fuel-extension-volume-manager,gitfred/fuel-extension-volume-manager