Implement cert cache for vault units (v4)

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.

This patch is mostly the same as
I18aa6c9193379ea454851b6f60a8f331ef88a980
but improved to avoid LP#1896542 by removing
the section where a certificate can be reused
from cache during create_certs.

Co-Authored-By: Rodrigo Barbieri <[email protected]>
Co-Authored-By: Alex Kavanagh <[email protected]>

func-test-pr: openstack-charmers/zaza-openstack-tests#1153

Closes-Bug: #1940549
Closes-Bug: #1983269
Closes-Bug: #1845961
Related-Bug: #1896542
Change-Id: I0cca13d2042d61ffc6a7c13eccb0ec8c292020c9
(cherry picked from commit 1a1953b)
(cherry picked from commit 56ca825)