private.pem

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