node-mixin: fix configuration for unset fsSelector/diskDeviceSelector

As per https://github.com/prometheus/node_exporter/pull/1429#discussion_r304210103
we want to fetch all devices and all fs types.

Currently, this is done by setting empty string which breaks most queries which rely on it.

This fixes it by setting the appropriate selector instead of empty string.

Signed-off-by: Sergiusz Urbaniak <sergiusz.urbaniak@gmail.com>

docs/node-mixin/config.libsonnet

@@ -9,13 +9,13 @@
     // empty, all filesystems are selected. If you have unusual
     // filesystem you don't want to include in dashboards and
     // alerting, you can exclude them here, e.g. 'fstype!="tmpfs"'.
-    fsSelector: '',
+    fsSelector: 'fstype!=""',
 
     // Select the device for disk-related queries. If left empty, all
     // devices are selected. If you have unusual devices you don't
     // want to include in dashboards and alerting, you can exclude
     // them here, e.g. 'device!="tmpfs"'.
-    diskDeviceSelector: '',
+    diskDeviceSelector: 'device!=""',
 
     // Some of the alerts are meant to fire if a critical failure of a
     // node is imminent (e.g. the disk is about to run full). In a