Fixes #6396: an error removing a DHCP record (record doesn't exist) stops the delete process for a host
Fixes #6412: Trying to remove a DHCP record fails due to an invalid subnet check
fixes #6583 - Use Enumerable#find instead of each where possible
fixes #6584 - Replace Kernel#open with File.open
fixes #6585 - Update Proxy::Util::CommandTask to handle Ruby 1.8 and 1.9+

In this update, use 'true' and 'false' to test $? return codes 0 and 1, respectively.
In Ruby 1.8.x, the return code from Open3.popen3 is always 0, some errors will not be caught.