Fixes #6412: Trying to remove a DHCP record fails due to an invalid subn...

...et check
Suggestion: replace the `subnet.records.each` with a `find`: http://ruby-doc.org/core-1.8.7/Enumerable.html#method-i-find

``` ruby
subnet.records.find do |v|
  v.options[:hostname] == hostname
env
```