Allow ssl_verify_mode to be configured

In Chef 11.12.0+, [insecure SSL verification mode triggers a warning](https://github.com/opscode/chef/blob/11-stable/RELEASE_NOTES.md#unsecure-ssl-verification-mode-now-triggers-a-warning) like this:

    SSL validation of HTTPS requests is disabled. HTTPS connections are still
    encrypted, but chef is not able to detect forged replies or man in the middle
    attacks.
    
    To fix this issue add an entry like this to your configuration file:
    
    ```
      # Verify all HTTPS connections (recommended)
      ssl_verify_mode :verify_peer
    
      # OR, Verify only connections to chef-server
      verify_api_cert true
    ```
    
    To check your SSL configuration, or troubleshoot errors, you can use the
    `knife ssl check` command like so:
    
    ```
      knife ssl check -c /home/vagrant/chef-solo/solo.rb
    ```

This pull request allows `ssl_verify_mode` to be configured.

When not configured, it defaults to `:verify_peer` to prevent the above warning. :smile: