Add source and readonly as allowed fields to the nfsbroker
Add source and readonly as allowed fields to the nfsbroker

[#160999930](https://www.pivotaltracker.com/story/show/160999930)

Co-authored-by: Paul Warren <381ebbe39e97b92332c450533d9eabb9a873db86@emc.com>

cloudfoundry-incubator/nfs-volume-release,cloudfoundry-incubator/nfs-volume-release,cloudfoundry-incubator/nfs-volume-release