Add $architecture parameter to mirror and repo resources

This change adds the option to create mirrors and repos with specific architectures.

```
  aptly::mirror {'puppetlabs-wheezy':
    location      => 'http://apt.puppetlabs.com',
    release       => 'wheezy',
    repos         => ['main', 'dependencies'],
    key           => '4BD6EC30',
    architectures => ['amd64','i386'],
  }
```
I wonder if `$archs_array` is a confusing name for a variable that doesn't contain an array. Maybe something like `$archs_concat` would be more descriptive?