[#64726964] Enable `domain`-specific YAML in hiera

Allows us to specify hiera values per set of mirrors.

Useful in the first instance to define one SSL certificate for
*.mirror.provider0.production.govuk.service.gov.uk and another for
*.mirror.provider1.production.govuk.service.gov.uk.
Merge pull request #29 from alphagov/64726964-hiera-domain-specific

[#64726964] Enable `domain`-specific YAML in hiera