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