Append newline to each key added

The change to this file 10 days ago removed a newline character at the end of each key added to it. This mean that when another key was added, it continued on the same line as the one before and thus wasn't being detected when an ssh connection came in with a key file.

With regards to https://github.com/mitchellh/vagrant/issues/7455 this is an (ugly) fix.

I'm sure someone knows a better command to concat a file and a string and append it to the `authorized_keys` file. But this does fix the problem.

plugins/guests/linux/cap/insert_public_key.rb

@@ -19,6 +19,7 @@ module VagrantPlugins
             mkdir -p ~/.ssh
             chmod 0700 ~/.ssh
             cat '#{remote_path}' >> ~/.ssh/authorized_keys
+            echo "\n" >> ~/.ssh/authorized_keys
             chmod 0600 ~/.ssh/authorized_keys
 
             # Remove the temporary file