Test DNS zone files

Summary:
Run sanity tests against roles/dns/knot/files/zones content:

• do we have zone files defined in pillar?
• can they be validated with kzonecheck?

Tests also validate the pillar files for content needed by zone files:

• is dns_identity defined?

Ref T1217

Test Plan: (cd _tests && make test-roles-dns)

Reviewers: DorianWinty

Maniphest Tasks: T1217

Differential Revision: https://devcentral.nasqueron.org/D3690