Use git.openstack.org everywhere
Use git.openstack.org everywhere

Our official git master is at git.openstack.org, update places
that use github instead.

Also fixes link to DefCore document, the document has moved.

Change-Id: Iac65643aa2566df2a24732f7b1c6d6093d8092f2

openstack/training-guides,openstack/training-guides