synchronize: quote private_key (ansible#21570)

otherwise rsync will fail when trying to access a key with spaces:

fatal: [default]: FAILED! => {"changed": false, "cmd": "/usr/bin/rsync --delay-updates -F --compress --archive --rsh 'ssh -i /home/user/Some Folder/.vagrant/machines/default/libvirt/private_key -S none -o StrictHostKeyChecking=no -o Port=22' --out-format='<<CHANGED>>%i %n%L' \"/etc/issue\" \"[email protected]:/tmp/issue2\"", "failed": true, "msg": "Warning: Identity file /home/user/Some not accessible: No such file or directory.\nssh: Could not resolve hostname folder/.vagrant/machines/default/libvirt/private_key: No address associated with hostname\r\nrsync: connection unexpectedly closed (0 bytes received so far) [sender]\nrsync error: unexplained error (code 255) at io.c(226) [sender=3.1.2]\n", "rc": 255}

lib/ansible/modules/files/synchronize.py

@@ -438,7 +438,7 @@ def main():
     if private_key is None:
         private_key = ''
     else:
-        private_key = '-i '+ private_key
+        private_key = '-i "%s"' % private_key
 
     ssh_opts = '-S none'