Fix Vagrant 1.4 compatibility and support multiple SSH keys

Vagrant 1.4.0 added support for multiple SSH keys (mitchellh/vagrant#907)
and changed `private_key_path` to an Array.

This commit ensures we always handle it as array to keep compatible with
both old and new Vagrant versions. Also pass all of the specified keys
to rsync/ssh command.

Fixes #171
Merge pull request #172 from tmatilai/multiple-ssh-keys

Fix Vagrant 1.4 compatibility and support multiple SSH keys