Fix #158: Add copytruncate to logrotate.conf.

From lograte man pages:

> `copytruncate`: Truncate the original log file to zero size in place after creating a copy, instead of moving the old log file and optionally creating a new one.

Why? Because file might be followed by file descriptior (and not name).
So when logrorate rotates the file, Prometheus will follow the old file
(thus receiving no metrics).

By using `comytruncate` we ensure that the followed file is always the
proper one.

helm/mail/values.yaml

@@ -52,6 +52,7 @@ metrics:
     enabled: true
     logrotate.conf: |
       /var/log/mail.log {
+          copytruncate
           rotate 1
           monthly
           minsize 1M