Only cache lookup for *edgeHost address

Because the hostname of the backend server used by the CDNBackendServer
helper tests isn't always "localhost" - it can be ::1 or 127.0.0.1 - which
results in those addresses being resolved to the same as *edgeHost and this
error occurs:

    helpers.go:141: dial tcp 172.16.20.10:8080: connection refused

We could use a more complex map of cached names and addresses. But I think
edgeHost is the only thing we want to cache.
Merge pull request #70 from alphagov/only_cache_edgehost

Only cache lookup for *edgeHost address