Issue title: Newbie question: timeout flag
 Issue body: This is probably a user error, but I can't get my script to run with the timeout flag.  I set up:
                register_dopar_cmq(n_jobs=self$nCores, memory=1024,
                                   fail_on_error=FALSE, timeout=Self$walltime)
And the template says:
#SBATCH --time={{ timeout }} 

And then I get this error:
Error in fill_template(private$template, opts, required = c("master",  :
  Template values required but not provided: timeout
In addition: There were 50 or more warnings (use warnings() to see the first 50)

What am I doing wrong?
 Comments: 
 Comment 0: It seems you created a custom template with a `{{ timeout }}` field that does not have a default value.

So, in order to be able to submit the job we need to fill this field. However, the `timeout` parameter to `Q` does something different (it is the time when we will assume a worker to be dead if we haven't heard from it) and it is not passed to your template.

So to solve this, you can pass it as a template argument:

```r
Q(..., template=list(timeout=...))
register_dopar_cmq(..., template=list(timeout=...))
```
 Comment 1: That was it, thanks!
{'source': 'GithubIssues', 'repo_name_with_owner': 'mschubert/clustermq', 'issue_id': 721900369, 'repo_stars': 129.0}