Add splay sleep before reboot nodes

Bug:
 Resolves concurrency issue when deploying
 large clusters. Uses formula of nodes /
 IOPS * 180 to determine how large the
 splay size should be.

Refactoring:
- speed up tests

Co-Authored-By: Matthew Mosesohn <mmosesohn@mirantis.com>

Depends on:
- https://review.openstack.org/#/c/113924/
- https://review.openstack.org/#/c/113920/

Change-Id: Ieefc843ccba0cdb0441679c73d2e39d6729a4fb0
Closes-Bug: #1355347