Add files verification for 'fuel2 graph upload' command

Now Nailgun reports about successful upload of deployment graph
from directory that can even not contain any 'tasks.yaml' files.
This patch fixes:
  * check if at least one 'tasks.yaml' file exists in specified
    directory path
  * check if at least one 'tasks.yaml' file in specified
    directory path is not empty

Change-Id: I5de2474b8fbb46780450ee3873af6189a6cf17cc
Closes-Bug: 1619586
(cherry picked from commit 9dfb28e33c)

fuelclient/commands/graph.py

@@ -80,10 +80,18 @@ class GraphUpload(base.BaseCommand, FileMethodsMixin):
 
         tasks = []
         for file_name in iterfiles(dir_path, 'tasks.yaml'):
-            tasks.extend(serializer.read_from_full_path(file_name))
+            task_data = serializer.read_from_full_path(file_name)
+            if task_data:
+                tasks.extend(task_data)
 
         if tasks:
             data['tasks'] = tasks
+
+        if not data:
+            msg = ("Nothing to upload. Check if at least one 'tasks.yaml' "
+                   "file is not empty and exists in '{path}' directory "
+                   "path".format(path=dir_path))
+            raise error.ActionException(msg)
         return data
 
     def get_parser(self, prog_name):

fuelclient/tests/unit/v2/cli/test_deployment_graph.py

@@ -18,6 +18,7 @@ import mock
 import six
 import yaml
 
+from fuelclient.cli import error
 from fuelclient.tests.unit.v2.cli import test_engine
 
 
@@ -148,6 +149,18 @@ class TestGraphActions(test_engine.BaseCLITest):
                 graph_type='provision'
             )
 
+    @mock.patch('fuelclient.commands.graph.os')
+    @mock.patch('fuelclient.commands.graph.iterfiles')
+    def test_graph_upload_from_dir_fail(self, iterfiles_m, os_m):
+        os_m.path.isdir.return_value = True
+        os_m.path.exists.side_effect = [True, False]
+        iterfiles_m.return_value = []
+        args = 'graph upload --release 1 --dir /graph/provision -t provision'
+
+        self.assertRaisesRegexp(error.ActionException,
+                                "Nothing to upload",
+                                self.exec_command, args)
+
     @mock.patch('sys.stderr')
     def test_upload_fail(self, mocked_stderr):
         cmd = 'graph upload --file new_graph.yaml -t test'