sync_folders: should have space between ssh-i options

This previous commit (a8bf7574de48dc1c62928f5ad73fb6d1e7dd4347) introduced a bug where the `-i` options passed to `rsync` are not properly white space delimited.

This will result in errors like:

```
Error: Warning: Identity file <<key>>-i not accessible: No such file or directory.
ssh: Could not resolve hostname <<key>>: nodename nor servname provided, or not known
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.0]
```

You would only manifest this error if you were to configure multiple ssh keys:

```
  config.ssh.private_key_path = [
    "#{ENV['HOME']}/.vagrant.d/insecure_private_key",
    "#{ENV['HOME']}/.ssh/<<key>>"
  ]
```