Reprocess options on a job_template

I wanted to do something like:

```ruby
set :job_template, "/usr/local/bin/lockrun --idempotent --lockfile=:lockfile -- /bin/bash -l -c ':job'"
```

However, because variables are only processed in creating the `:job`, and `:job_template` is only processed using `:job`, `:lockfile` was ignored. This will also allow some further shortening of base templates:

```ruby
job_type :rake, ":environment_variable=:environment bundle exec rake :task --silent :output"
set :job_template, 'cd :path && :job'
```

No tests yet—I'm floating the idea first because I think it would be useful.