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/change-password-symmetric-vault.yml

@@ -0,0 +1,18 @@
+---
+- name: Playbook to change password of symmetric vault.
+  hosts: ipaserver
+  become: yes
+  gather_facts: no
+
+  tasks:
+  - name: Create vault.
+    ipavault:
+      ipaadmin_password: SomeADMINpassword
+      name: symvault
+      password: SomeVAULTpassword
+  - name: Change vault passord.
+    ipavault:
+      ipaadmin_password: SomeADMINpassword
+      name: symvault
+      password: SomeVAULTpassword
+      new_password: SomeNEWpassword