apt::source templates/sources.list.erb generates invalid source line when architecture is provided.

When trying to add the CDH4 repository to APT I ran into the following problem:

Using the following 

```puppet                                                           
    apt::source {'cdh4':                                                                                                                        
        location => 'http://archive.cloudera.com/cdh4/ubuntu/precise/amd64/cdh',          
        release  => 'precise-cdh4',                                                       
        repos => 'contrib',                                                               
        architecture => 'amd64',                                                          
        include_src => false,                                                             
        key => '02A818DD',                                                                
        key_server => 'keys.gnupg.net'                                                    
    }                                                                                              
```

Generates /etc/apt/sources.list.d/cdh4.list containing:

```
# cdh4
deb [arch=amd64]http://archive.cloudera.com/cdh4/ubuntu/precise/amd64/cdh precise-cdh4 contrib
```

Notice there is a missing space between [arch=amd64] and http:///...

The source of this bug is in the templates/sources.list.erb file which is not adding a space in the optional architecture statement.