Implement vault encrypted yaml variables. (#16274)

Make !vault-encrypted create a AnsibleVaultUnicode
yaml object that can be used as a regular string object.

This allows a playbook to include a encrypted vault
blob for the value of a yaml variable. A 'secret_password'
variable can have it's value encrypted instead of having
to vault encrypt an entire vars file.

Add __ENCRYPTED__ to the vault yaml types so
template.Template can treat it similar
to __UNSAFE__ flags.

vault.VaultLib api changes:
    - Split VaultLib.encrypt to encrypt and encrypt_bytestring

    - VaultLib.encrypt() previously accepted the plaintext data
      as either a byte string or a unicode string.
      Doing the right thing based on the input type would fail
      on py3 if given a arg of type 'bytes'. To simplify the
      API, vaultlib.encrypt() now assumes input plaintext is a
      py2 unicode or py3 str. It will encode to utf-8 then call
      the new encrypt_bytestring(). The new methods are less
      ambiguous.

    - moved VaultLib.is_encrypted logic to vault module scope
      and split to is_encrypted() and is_encrypted_file().

Add a test/unit/mock/yaml_helper.py
It has some helpers for testing parsing/yaml

Integration tests added as roles test_vault and test_vault_embedded

test/units/parsing/test_data_loader.py

@@ -20,14 +20,12 @@ from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
 from six import PY3
-from yaml.scanner import ScannerError
 
 from ansible.compat.tests import unittest
 from ansible.compat.tests.mock import patch, mock_open
 from ansible.errors import AnsibleParserError
 
 from ansible.parsing.dataloader import DataLoader
-from ansible.parsing.yaml.objects import AnsibleMapping
 
 class TestDataLoader(unittest.TestCase):
 
@@ -85,6 +83,6 @@ class TestDataLoaderWithVault(unittest.TestCase):
         else:
             builtins_name = '__builtin__'
 
-        with patch(builtins_name + '.open', mock_open(read_data=vaulted_data)):
+        with patch(builtins_name + '.open', mock_open(read_data=vaulted_data.encode('utf-8'))):
             output = self._loader.load_from_file('dummy_vault.txt')
             self.assertEqual(output, dict(foo='bar'))