This cache is used to store certificates and keys
issued by the leader unit. Non-leader units read
these certificates and keep data in their
"tls-certificates" relations up to date.
This ensures that charm units that receive certs
from vault can read from relation data of any
vault unit and receive correct data.
Closes-Bug: #1940549
Change-Id: Iac989b30948fa43fe23851995a8ed00b08126587