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/integration/roles/test_vault_embedded/tasks/main.yml

@@ -0,0 +1,14 @@
+---
+- name: Assert that a embedded vault of a string with no newline works
+  assert:
+    that:
+      - '"{{ vault_encrypted_one_line_var }}" == "Setec Astronomy"'
+
+- name: Assert that a multi line embedded vault works, including new line
+  assert:
+    that:
+      - vault_encrypted_var == "Setec Astronomy\n"
+
+# TODO: add a expected fail here
+# - debug: var=vault_encrypted_one_line_var_with_embedded_template
+