sample command output should be wrapped in parenthesis

With:

``` cwl
cwlVersion: v1.0
class: CommandLineTool

inputs:
my_directory: Directory

baseCommand: [bash, -c]

arguments:
- "ls $(inputs.my_directory.path); ls $(inputs.my_directory.path)"

outputs:
result: stdout
```

Current:
```
$ cwltool test.cwl
INFO /usr/bin/cwltool 1.0.20190915164430
INFO Resolved 'test.cwl' to 'file:///home/michael/test.cwl'
INFO [job test.cwl] /tmp/tw9azxpv$ bash \
-c \
'ls /tmp/tmpgu2ttti2/stgddcf5c1b-4e40-4992-bf48-ce01f40b6df2/t; ls /tmp/tmpgu2ttti2/stgddcf5c1b-4e40-4992-bf48-ce01f40b6df2/t' > /tmp/tw9azxpv/da429be172539cb76be1c4634e042936a95faf6a
INFO [job test.cwl] completed success
{
"result": {
"location": "file:///home/michael/da429be172539cb76be1c4634e042936a95faf6a",
"basename": "da429be172539cb76be1c4634e042936a95faf6a",
"class": "File",
"checksum": "sha1$8fa3e78e5eab5a0273c714ec57ba79bc5b046bba",
"size": 8,
"path": "/home/michael/da429be172539cb76be1c4634e042936a95faf6a"
}
}
INFO Final process status is success

```

Desired:

```
$ cwltool test.cwl
INFO /usr/bin/cwltool 1.0.20190915164430
INFO Resolved 'test.cwl' to 'file:///home/michael/test.cwl'
INFO [job test.cwl] /tmp/tw9azxpv$ ( \
bash \
-c \
'ls /tmp/tmpgu2ttti2/stgddcf5c1b-4e40-4992-bf48-ce01f40b6df2/t; ls /tmp/tmpgu2ttti2/stgddcf5c1b-4e40-4992-bf48-ce01f40b6df2/t' \
) > /tmp/tw9azxpv/da429be172539cb76be1c4634e042936a95faf6a
INFO [job test.cwl] completed success
{
"result": {
"location": "file:///home/michael/da429be172539cb76be1c4634e042936a95faf6a",
"basename": "da429be172539cb76be1c4634e042936a95faf6a",
"class": "File",
"checksum": "sha1$8fa3e78e5eab5a0273c714ec57ba79bc5b046bba",
"size": 8,
"path": "/home/michael//da429be172539cb76be1c4634e042936a95faf6a"
}
}
INFO Final process status is success
```