Make sure synced folder types are always symbols

This will make sure that plugins that define synced folders after this
config object has been finalized won't have trouble because of
forgetting to use symbols instead of strings.

References https://github.com/fgrehm/vagrant-cachier/issues/76
Merge pull request #2925 from fgrehm/ensure-synced-folder-types-are-symbols

kernel/v2: Make sure synced folder types are always symbols internally