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-symmetric-vault.yml

@@ -1,8 +1,8 @@
 ---
 - name: Tests
   hosts: ipaserver
-  become: true
-  gather_facts: True
+  become: no
+  gather_facts: no
 
   tasks:
     - name: Retrieve data from symmetric vault.
@@ -11,8 +11,7 @@
         name: symvault
         username: admin
         password: SomeVAULTpassword
-        retrieve: yes
-        action: member
+        state: retrieved
       register: result
     - debug:
         msg: "{{ result.data | b64decode }}"