Wait for local file systems before starting prometheus (#317)

In case prometheus data is stored on mounted volume, wait on it before
starting process.

This change also makes sure that prometheus is stopped to allow safe
unmount volume during shutdown.

[patch] release

Co-authored-by: Bartosz Cisek <bartosz.cisek@ebury.com>

templates/prometheus.service.j2

@@ -3,6 +3,8 @@
 [Unit]
 Description=Prometheus
 After=network-online.target
+Requires=local-fs.target
+After=local-fs.target
 
 [Service]
 Type=simple