Move the slugs for No10 and DPMO

Separate Puppet pull request to come with redirection. This needs to be
released first.

https://www.pivotaltracker.com/story/show/47364021
Merge pull request #457 from alphagov/move-executive-offices

Move the slugs for No10 and DPMO

https://www.pivotaltracker.com/story/show/47364021