Add template locals variables to job_template

Use Case: Run crons with [lockrun](http://www.unixwiz.net/tools/lockrun.html).

`/path/to/lockrun --lockfile=/tmp/:lockrun_id.lockrun -- sh -c 'some command'`

This pull-request adds the ability to interpolate variables in a job_template.

```ruby
set :job_template, "/path/to/lockrun --lockfile=/tmp/:lockrun_id.lockrun -- sh -c ':job'"

every 1.day, :at => '5:00 am' do
  rake "foo:bar", template_locals: {:lockrun_id => 'ID'}
end

# Without this patch, the output would be:
# => 0 * * * * /usr/bin/lockrun --lockfile=/tmp/:lockrun_id.lockrun -- sh -c ' command '

# With this patch, the output would be:
# => 0 * * * * /usr/bin/lockrun --lockfile=/tmp/ID.lockrun -- sh -c ' command '
```