In some cases nodepool-launcher uses public API to launch nodes, but
doesn't have access to the private networks of nodes it launches.
Rather then failing, expose an option for operators to disable
ssh-keyscan and allow nodes to become ready.
Change-Id: I764398aa21461ef44048e9e6565d2ee3e01aaaf8
Signed-off-by: Paul Belanger <pabelanger@redhat.com>