Fixes password behavior on Vault module.

This patch fixes handling of password and public_key files, parameter
validation depending on vault type, usage of `salt` attribute and data
retrieval.

Tests were updated to reflect the changes.

New example playbooks are added:

    playbooks/vault/vault-is-present-with-password-file.yml
    playbooks/vault/vault-is-present-with-public-key-file.yml
    playbooks/vault/retrive-data-asymmetric-vault.yml
    playbooks/vault/retrive-data-symmetric-vault.yml

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

@@ -0,0 +1,18 @@
+---
+- name: Tests
+  hosts: ipaserver
+  become: true
+  gather_facts: True
+
+  tasks:
+    - name: Retrieve data from symmetric vault.
+      ipavault:
+        ipaadmin_password: SomeADMINpassword
+        name: symvault
+        username: admin
+        password: SomeVAULTpassword
+        retrieve: yes
+        action: member
+      register: result
+    - debug:
+        msg: "{{ result.data | b64decode }}"