Always set an aliases value to pass to nginx config template

Summary:
When a container doesn't have any alias for hostname,
an empty string is passed to the nginx configuration template.

This is a follow-up for af0a1983.

Test Plan:
Run salt equatower state.apply roles/paas-docker/nginx/config
then look the Phabricator containers vhosts (only DevCentral has aliases).

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1926