notification-http: Use environment proxy settings (#4364)

Make use of the http_proxy, https_proxy, and no_proxy environment
variables (and their uppercase equivalents) when sending http
notifications.

Author: op3

cmd/notification-http/main.go

@@ -95,6 +95,7 @@ func getTLSClient(c *PluginConfig) error {
 	}
 
 	transport := &http.Transport{
+		Proxy:           http.ProxyFromEnvironment,
 		TLSClientConfig: tlsConfig,
 	}