Make sure ~/.ssh/known_hosts exists before calling ssh-keygen -R
Make sure ~/.ssh/known_hosts exists before calling ssh-keygen -R

Our vagrant file makes use of ssh-keygen -R to remove any old key
related to an IP address from known_hosts.

But ssh-keygen fails and makes the local.sh exit with failure
(error 2) if the ~/.ssh/known_hosts file does not exist.

Change-Id: I404e79ca644c6c15f51a0f3db6695cc5a895b238

openstack/octavia,openstack/octavia,openstack/octavia