Fixes README for training-labs
Fixes README for training-labs

Removes unnecessary content from README.md and adds basic introduction about
training-labs folder.

blueprint openstack-training-labs

Change-Id: Id3a1fb7db838a23904c38d69d29403f656e13783

Nabs007/training-guides,openstack/training-guides,harrisonfeng/training-guides,harrisonfeng/training-guides,openstack/training-guides,harrisonfeng/training-guides,Nabs007/training-guides,Nabs007/training-guides