remove openstack related document

Remove OpenStack related documents except using_with_openstack.
README of Quantum Ryu plugin refers to it.

remove:
  doc/source/images/*
  doc/source/openstack.rst
  doc/source/step_by_step.rst
  doc/source/internals_l2_isolation.rst

modified:
  doc/source/index.rst
    remove index of openstack.rst.
  doc/source/using_with_openstack.rst
    replace contents with the pointer to the wiki page.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>