Stress on the fact the domain is a fully qualified one (ii)

Summary:
Like in 06ff5d8a5ba6, rename domain into fqdn
per https://agora.nasqueron.org/Code_conventions#Salt

This is a follow-up for 88ad7d65.

Test Plan: no op

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1915