Integrates Vault PKI service as a service addon.
Also adds some helper scripts to help setup the CA in Vault, as well as
some docs to explain how to use them.
Originally based from https://github.com/mteodor/vault.
Signed-off-by: Joao Matos <joao@tritao.eu>
Co-authored-by: Dušan Borovčanin <dusan.borovcanin@mainflux.com>
