depend on time-sync.target

Cargo-culted from Debian, but it makes sense that Prometheus should
start only after a proper time sync has been achieved, considering how
important timestamps are in its architecture.

roles/prometheus/templates/prometheus.service.j2

@@ -2,9 +2,7 @@
 
 [Unit]
 Description=Prometheus
-After=network-online.target
-Requires=local-fs.target
-After=local-fs.target
+After=time-sync.target
 
 [Service]
 Restart=on-failure