NFS responses contain undescribed performance_tier property

When querying for NFS's, e.g.:

/v2/nfs
/v2/nfs/{nfs_id}

The response will contain a property performance_tier which is not described in the documentation. This property will contain a value such as `PERFORMANCE_TIER_STANDARD`. Since this value is different from the corresponding value passed in NFS-requests (which would be `standard`), this cause some confusion and leads to bugs. Such as in the DigitalOcean terraform provider.