system: filesystems in exclude are excluded from content analysis

some disks can be very slow and noisy (at home). this allows users to simply skip them.
also, applicable for large storage boxes

src/routes/test/system-test.js

@@ -130,7 +130,7 @@ describe('System', function () {
 
     describe('disk usage', function () {
         it('get succeeds with no cache', async function () {
-            safe.fs.unlinkSync(paths.DISK_USAGE_FILE);
+            safe.fs.unlinkSync(paths.DISK_USAGE_CACHE_FILE);
 
             const response = await superagent.get(`${serverUrl}/api/v1/system/disk_usage`)
                 .query({ access_token: owner.token })