Expand `config[:synced_folders]` paths before genarating Vagrantfile

Relative source paths were written as is, leading to paths relative to
the Vagrantfile. This expands them to relative to the kitchen root.

Also move `%{instance_name}` and option expanding out from the
Vagrantfile template so that custom templates don't need to copy the
logic.

Fixes: #70
Improve `synced_folders` documentation

Modify the example to use relative path and `%{instance_name}`.
Merge pull request #72 from tmatilai/relative_synced_folders

Fix relative synced folder paths