Enable customization of VMDK type with `disk_type_id` parameter

This PR enables the `disk_type_id` parameter for the VMware builder, allowing customization of the underlying disk type used.  The default remains '1' (growable virtual disk split in 2GB files).

A string type was used here instead of an integer because `mapstructure` defaults `uint` to 0, which would be a valid disk type id.