Add hashi_vault pki backend support

This patch defines necessary variables for the ‘hashi_vault’ pki backend which is the alternative to the default ‘standalone’ backend.

Additionally, it:

• passes pki_default_backend when trigerring PKI role so its aware of the default backend in case it’s not explicitly specified in the cert definition.
• adopts this role to the recent changes in PKI role [2][3][4]