Add blueprint for configurable ssh host key policy

This spec proposes adding a blueprint to make the way that
ssh host keys are handled configurable so that administrators
can choose the level of ssh security they have.

Change-Id: I734882582cc73be0a10a8571e130a9a797bbd41d
blueprint: configurable-ssh-host-key-policy
Merge "Add blueprint for configurable ssh host key policy"