Vagrant config customization: support for synced folder options

- [x] Add support to override synced folder options
- [x] Migrate customization to plain-ruby settings file instead of YAML

~~One open question: does it make sense to switch synced options to defaults instead of forcing nfs for non-windows. Currently on Linux host default is vbox shared folders - seems working and gradle build passes; I'm yet to check for performance regressions and to test VM rebuild from scratch .~~
 - The build takes longer with vbox shares compared to nfs. Scratching that for now.