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/tasks/main.yml

@@ -0,0 +1,7 @@
+
+- assert:
+    that:
+      - 'secret_var == "secret"'
+
+
+- copy: src=vault-secret.txt dest={{output_dir}}/secret.txt

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

@@ -0,0 +1,9 @@
+$ANSIBLE_VAULT;1.1;AES256
+31626536666232643662346539623662393436386162643439643434656231343435653936343235
+6139346364396166336636383734333430373763336434310a303137623539653939336132626234
+64613232396532313731313935333433353330666466646663303233323331636234326464643166
+6538653264636166370a613161313064653566323037393962643032353230396536313865326362
+34396262303130326632623162623230346238633932393938393766313036643835613936356233
+33323730373331386337353339613165373064323134343930333031623036326164353534646631
+31313963666234623731316238656233396638643331306231373539643039383434373035306233
+30386230363730643561