Configure ViperServ eggdrops to use Vault

Summary:
Eggdrop configuration

Load Vault library and helper script.

Use Vault to connect to MySQL and to fetch nickserv password.

Salt deployment

Provide a policy allowing to read apps/viperserv/

Provision AppRole credentials in .credentials file, and remove
MySQL information placeholder as it's known stored in Vault.

Ref T1733

Test Plan: Launch eggdrop

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1733

Differential Revision: https://devcentral.nasqueron.org/D2687