Added nsswitch so that /etc/hosts file will work.

roles/prom_server/templates/prometheus.service

@@ -11,6 +11,7 @@ ExecStartPre=-/usr/bin/docker rm %n
 ExecStart=/usr/bin/docker run --name %n \
     --network host \
     -v /srv/prometheus/prometheus:/prometheus  \
+    -v /srv/prometheus/etc/nsswitch.conf/etc/nsswitch.conf  \
     -v /srv/prometheus/etc/prometheus:/etc/prometheus  \
     prom/prometheus:v2.6.0 \
     --storage.tsdb.retention 365d --config.file=/etc/prometheus/prometheus.yml \