Add support for password as docker secrets

[m0rtalis]

Hi,
would like to include the mangement api key as docker secret into the container instead of having to expose the password in the environments section of my docker-compose file.

I would imaging using an environments variable like OPENVPNMONITOR_SITES_<INDEX>_PASSWORD_FILE=/run/secrets/mngmt-key to point to the secret.

Since conf.d does not support multiple backends we would need to iterate the environment variables as shown here and call a function like this to expose the correct env-var before calling confd in entrypoint.sh.
Also the key should be ignored in confd/templates/openvpn-monitor.conf.tmpl#L25 (or it needs a name not starting with OPENVPNMONITOR_SITES).

docker-openvpn-monitor/confd/templates/openvpn-monitor.conf.tmpl

Lines 24 to 26 in 889b784

	show_disconnect={{- getv (printf "/openvpnmonitor/sites/%s/%s" $i "showdisconnect") -}}
	{{- else -}}
	{{- $property }}={{- getv (printf "/openvpnmonitor/sites/%s/%s" $i $property) -}}

I can create a PR for this if you want to implement it as suggested.

[ruimarinho]

That would be great, @m0rtalis!

[mrquincle]

How to actually write the password in the environment section? What's the syntax?