Replace df plugin with custom df plugin

The built-in df plugin cannot do the following:
    * if we choose by type ext4, we want to skip devicemapper (on scaleway)
    * the MountPoint of the appsdata directory is not possible to know at install time

Fixes #398

webadmin/src/views/graphs.js

@@ -32,6 +32,7 @@ angular.module('Application').controller('GraphsController', ['$scope', '$locati
     }
 
     function renderDisk(type, free, reserved, used) {
+        // this will mismatch df output since df -H is SI units (1000)
         $scope.diskUsage[type] = {
             used: bytesToGigaBytes(used.datapoints[0][0] + reserved.datapoints[0][0]),
             free: bytesToGigaBytes(free.datapoints[0][0]),