[201811] Invoke disk check periodically (#8951)

* Invoke disk check periodically. (#7374)

Why I did it
Helps with periodic scan of disk for RO state.
If found, this script makes transient fix and raise error message.

files/image_config/monit/conf.d/sonic-host

@@ -20,3 +20,10 @@ check process rsyslog with pidfile /var/run/rsyslogd.pid
     start program = "/bin/systemctl start rsyslog.service"
     stop program = "/bin/systemctl stop rsyslog.service"
     if totalmem > 800 MB for 10 times within 20 cycles then restart
+
+# Check if /etc & /home are writable. If not, make them writable.
+# Raise syslog error message, in case of underlying issues
+#
+check program diskCheck with path "/usr/local/bin/disk_check.py"
+    if status != 0 then alert
+