Add links to summit videos in user/cells.rst
Add links to summit videos in user/cells.rst

This patch is a follow-up for
I66a2adb3ff75da6e267536f25c2eda5925f2fa87.

Add links to videos recorded at Rocky or Stein summit
in doc/source/user/cells.rst.

Change-Id: Idcc77cf2eee809f3ca99f952f0635213f3bb78eb

klmitch/nova,openstack/nova,klmitch/nova,mikalstill/nova,rahulunair/nova,klmitch/nova,rahulunair/nova,openstack/nova,openstack/nova,mahak/nova,mahak/nova,rahulunair/nova,mahak/nova,mikalstill/nova,mikalstill/nova,klmitch/nova