[SET-651] Adds support to read secrets from HashiCorp Vault

[gaol]

Issue: https://issues.redhat.com/browse/SET-651

This would be the first PR to add the support to read secrets from a specified HashiCorp Vault instance.

What it does are:

• Adds a role called vault to read secrets from specified paths in a HashiCorp Vault server. It uses the secret key as the Ansible variable name, and uses the secret value as the Ansbile variable value.
  • The role supports both token and approle authentication method, depending on the environment set up, it will use different authentication approach.
• Adds community.hashi_vault to the requirements.yml file
• Update to ssh role to support writing ssh private key files from content besides copying from another file. This is because the private keys are secrets, and they need to be put somewhere in the HashiCorp Vault. The change won't change current behavior unless the variable get updated (ssh_keys -> ssh_keys_content).
• Adds the molecule tests for vault role

The community.hashi_vault requires hvac python library installed, I don't add that in this PR because I think it is belongs to the playbook change, unless the vault role is used, it is not required.

The subsequent PR will include the vault role as the first task in the pre_tasks section in the playbooks to enable vault access.

[gaol]

@rpelisse would you please review ? thanks

[rpelisse]

LGTM!