Add futures library to global requirements

The concurrent.futures lib will be used to implement parallel file
downloads in Ironic project. Basing on discussion in
https://review.openstack.org/#/c/63904/1 we decided to use futures
instead of working with Thread directly. Add 2.1.3 as minimal
version because the docs used during the development are for 2.1.3.

Change-Id: I94f8b5225ed380251a6d2e4ff6889c1c1bcb5b57
Merge "Add futures library to global requirements"