Automatic merge from submit-queue

examples/volumes/flexvolume/nfs: check for jq and simplify quoting

**What this PR does / why we need it**:

This PR improves error reporting of the nfs flex plugin script. Before it output non-JSON error, when `jq` wasn't installed on the server:
```
$ /usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s~nfs/nfs getvolumename test
/usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s~nfs/nfs: line 87: jq: command not found
/usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s~nfs/nfs: line 88: jq: command not found
{"status": "Success", "volumeName": "/"}
```
K8s fails to unmarshal such output to show an error in the logs:
>E0606 19:32:30.196262   25700 driver-call.go:212] Failed to unmarshal output for command: getvolumename, output: "/usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s\~nfs/nfs: line 87: jq: command not found\n/usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s\~nfs/nfs: line 88: jq: command not found\n{\"status\": \"Success\", \"volumeName\": \"/\"}", error: invalid character '/' looking for beginning of value

After  this change:
```sh
$ /usr/libexec/kubernetes/kubelet-plugins/volume/exec/k8s~nfs/nfs getvolumename test
{ "status": "Failure", "message": "'jq' binary not found. Please install jq package before using this driver"}
```
and
> E0606 19:52:07.915594   25700 driver-call.go:219] getvolumename command failed, status: Failure, reason: 'jq' binary not found. Please install jq package before using this driver

Also this PR improves quoting a bit by using single quotes where it possible.

**Release note**:
```release-note
NONE
```

CC @mfojtik