Fixed nfs-client Makefile, so that it builds on osx

[ianchakeres]

Make fails on osx with the following error:

cd nfs-client; \
	make container
CGO_ENABLED=0 go build -o docker/x86_64/nfs-client-provisioner ./cmd/nfs-client-provisioner
sudo docker build -t quay.io/external_storage/nfs-client-provisioner:latest docker/x86_64
Password:

This PR updates the Makefile so that the nfs-client-provisioner builds on osx.

I removed the unnecessary sudo before the docker commands.

Additionally, I added GOOS=linux and compiler arguments to statically link the binaries in the build commands.

[ianchakeres]

/assign @wongma7 @salkin

[k8s-ci-robot]

@ianchakeres: GitHub didn't allow me to assign the following users: salkin.

Note that only kubernetes-incubator members and repo collaborators can be assigned.

In response to this:

/assign @wongma7 @salkin

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.