Add state retrieved to ipavault to retrieve vault stored data.

This patch adds support for retrieving data stored in an IPA vault by
adding a new valid state for ipavault: `retrieved`.

To allow the retrieval of data from assymetric vaults, the attributes
`private_key`, `private_key_files` and `out` were also added to the
module.

The private key files, `private.pem`, should be paired with the already
existing `public.pem` public key files.

Tests were updated to reflect changes and two new playbooks were added:

    playbooks/vault/retrive-data-asymmetric-vault.yml
    playbooks/vault/retrive-data-symmetric-vault.yml

playbooks/vault/retrive-data-asymmetric-vault.yml

@@ -1,19 +1,17 @@
 ---
 - name: Tests
   hosts: ipaserver
-  become: true
-  gather_facts: True
+  become: no
+  gather_facts: no
 
   tasks:
     - name: Retrieve data from assymetric vault with a private key file.
       ipavault:
         ipaadmin_password: SomeADMINpassword
-        name: symvault
-        username: admin
+        name: asymvault
+        username: user01
         private_key_file: private.pem
-        retrieve: True
+        state: retrieved
       register: result
     - debug:
        msg: "Data: {{ result.data }}"
-    - debug:
-       msg: "Decoded Data: {{ result.data | b64decode }}"