Update nfsv3driver start script to use exec
Update nfsv3driver start script to use exec

- to fix an issue when monit stopped

[#161522289](https://www.pivotaltracker.com/story/show/161522289)

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

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