Add host keys to quantum for nexus plugin

The nexus plugin is executing as the quantum
user, whose home dir is /var/lib/quantum.

This patch will place the host key for each
nexus switch in /var/lib/quantum/.ssh/known_hosts
to prevent the plugin from failing to authenticate.
Merge pull request #120 from michaeltchapman/fix_nexus_ssh

Add host keys to quantum for nexus plugin