Add custom_repo variable to override passenger yum repo.

Added custom_repo so the yum repository can be disabled.  Foreman 1.2 includes passenger in the repository.  This allows the module to be backwards compatible with Foreman 1.1 and earlier, while also giving users the ability to have a custom repo.

I've also updated foreman and will create a pull for that if this is approved.

```
  include apache::ssl
  class { '::passenger':
    custom_repo => true,
  }
```