[DNS] Added support for Fog::DNS::Softlayer::Records.get

Added support to get a new domain record to keep compatibility with Fog

**Usage:**
If you would like to get record id: 123456, you can get it directly from service:

```ruby
@service = Fog::DNS[:softlayer]
@record = @service.records.get(123456)
 ```

or if you have a @domain you can get:
```ruby
@domain.records.get(123456)
 ```

#save operation will work on both cases
this test confirms that record will not require :domain as removed on https://github.com/softlayer/fog-softlayer/pull/3/files#diff-9caa7875eace39b9bc9e66ca7e72823bL26