roles/matrix-synapse: got federation working

This is beacause nginx will do canonization of URIs before
proxy-passing if you add anything after the port host:port.

For example, with:

     location /anything {
          proxy_pass http://127.0.0.1:8008/anything;
     }

NGINX will perform canonization, but with:

     location /anything {
          proxy_pass http://127.0.0.1:8008
	  # With NO Trailing slash
     }

NGINX will pass the unaltered original request, included /anything, to
the upstream server.

Canonization leads to problem when federating to other matrix-synapse
instances. The observed bug was:

- User A on our server (@A:our.server) invites User B on external
server (@B:matrix.org).

- User B correctly receive the invite.

- User B try to join the room but the request timeout.

- On our.sever logs we see "signature tampered" errors related to
  incoming connection from the external server (matrix.org).

This commit fix the issue.

roles/matrix-synapse/defaults/main.yaml

@@ -1,7 +1,7 @@
 ---
 server_fqdn: '{{ ansible_hostname }}.{{ domain }}'
 proxy_location_path: "_matrix"
-remote_host: "http://127.0.0.1:8008/_matrix"
+remote_host: "http://127.0.0.1:8008"
 matrix_domain: '{{ domain }}'
 ldap_server: 'ldap1.dmz.{{ domain }}'
 ldap_domain: '{{ domain }}'