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
+

test/integration/roles/test_vault_embedded/vars/main.yml

@@ -0,0 +1,17 @@
+# If you use normal 'ansible-vault create' or edit, files always have at least one new line
+# so c&p from a vault encrypted that wasn't specifically created sans new line ends up with one.
+# (specifically created, as in 'echo -n "just one line" > my_secret.yml'
+vault_encrypted_var: !vault-encrypted |
+              $ANSIBLE_VAULT;1.1;AES256
+              66386439653236336462626566653063336164663966303231363934653561363964363833313662
+              6431626536303530376336343832656537303632313433360a626438346336353331386135323734
+              62656361653630373231613662633962316233633936396165386439616533353965373339616234
+              3430613539666330390a313736323265656432366236633330313963326365653937323833366536
+              34623731376664623134383463316265643436343438623266623965636363326136
+vault_encrypted_one_line_var: !vault-encrypted |
+                              $ANSIBLE_VAULT;1.1;AES256
+                              33363965326261303234626463623963633531343539616138316433353830356566396130353436
+                              3562643163366231316662386565383735653432386435610a306664636137376132643732393835
+                              63383038383730306639353234326630666539346233376330303938323639306661313032396437
+                              6233623062366136310a633866373936313238333730653739323461656662303864663666653563
+                              3138