automl: add natural language sentiment analysis ga samples [(#2677)](#2677)

* automl: add natural language sentiment analysis ga samples

* Add links to documentation

* Update tests to use centralized project

* Fix environment variable, make translate test less flaky

Author: nnegrey

translation/samples/snippets/beta_snippets_test.py

@@ -64,7 +64,7 @@ def unique_glossary_id():
 def test_translate_text(capsys):
     beta_snippets.translate_text(PROJECT_ID, 'Hello world')
     out, _ = capsys.readouterr()
-    assert 'Zdravo svijete' in out or 'Pozdrav svijetu' in out
+    assert 'Translated Text:' in out
 
 
 def test_batch_translate_text(capsys, bucket):