Update Sentinel systemd file with more refined dependencies

deployment/ansible-role-sentinel/templates/sentinel.service.j2

 [Unit]
 Description=A flexible database for next generation sequencing metrics
+After=network-online.target
 
 [Service]
 ExecStart=/usr/bin/java -Dconfig.file={{ sentinel_dep_conf }} -jar {{ sentinel_dep_jar }}
@@ -14,5 +15,9 @@ RestartSec=1s
 StartLimitInterval=20s
 StartLimitBurst=5
 
+[Path]
+PathExists={{ sentinel_dep_conf }}
+PathExists={{ sentinel_dep_jar }}
+
 [Install]
 WantedBy=multi-user.target