Document minimum ansible version (2.7.6)

[sseago]

Currently, ocp3 clusters fail to install properly with ansible 2.6.x. agnosticd issue will also be filed, so at this point it's not clear whether this is a bug to be fixed (so this version restriction would be temporary) or whether 2.6.x is not fully supported. With ansible 2.6.18, we have observed that the reboot task for nodes (after installing updated kernel packages) is not working, although it is reporting success. The issue does not happen with 2.8.0, and it appears to be fine with 2.7.6 as well.

For now we should just add a README note to identify 2.7.6 as the minimum ansible version supported. We can remove the restriction if the agnosticd resolution is to fix the problem.

[sseago]

Here's the upstream issue I opened: redhat-cop/agnosticd#512