synced_folders/rsync: add UserKnownHostsFile to /dev/null [GH-3511]

CHANGELOG.md

@@ -53,6 +53,7 @@ BUG FIXES:
 
   - synced\_folders/nfs: Acquires a process-level lock so exports don't
       collide with Vagrant running in parallel.
+  - synced\_folders/rsync: Add UserKnownHostsFile option to not complain. [GH-3511]
 
 PLUGIN AUTHOR CHANGES:
 

plugins/synced_folders/rsync/helper.rb

@@ -57,7 +57,9 @@ module VagrantPlugins
         username = ssh_info[:username]
         host     = ssh_info[:host]
         rsh = [
-          "ssh -p #{ssh_info[:port]} -o StrictHostKeyChecking=no",
+          "ssh -p #{ssh_info[:port]} " +
+          "-o StrictHostKeyChecking=no " +
+          "-o UserKnownHostsFile=/dev/null",
           ssh_info[:private_key_path].map { |p| "-i '#{p}'" },
         ].flatten.join(" ")