Fix the letsencrypt CA.

See gluster/glusterfs-debian#4

In short, we serve the certificate and the CA, and apt do not like
it. So using only the chain.pem instead of fullchain.pem
should be sufficient.

tasks/letsencrypt.yml

@@ -49,4 +49,4 @@
   with_items:
   - { src: 'privkey.pem',   dst: 'private/{{ _website_domain }}.key' }
   - { src: 'cert.pem',      dst: 'certs/{{ _website_domain }}.crt' }
-  - { src: 'fullchain.pem', dst: 'certs/{{ _website_domain }}.ca.crt' }
+  - { src: 'chain.pem', dst: 'certs/{{ _website_domain }}.ca.crt' }