Recent backport breaks with many certificates (mplement cert cache for vault units (v2))

A recent backport to vault breaks when vault manages many certificates for charm clients:

2022-07-30 01:23:36 DEBUG unit.vault/1.juju-log server.go:319 certificates:95: Saving certificate for "gnocchi_0" (cn: "gnocchi.silo2.solutionsqa") into cache.
2022-07-30 01:23:36 ERROR unit.vault/1.juju-log server.go:319 certificates:95: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charmhelpers/core/hookenv.py", line 1180, in inner_translate_exc2
    return f(*args, **kwargs)
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charmhelpers/core/hookenv.py", line 1241, in leader_set
    subprocess.check_call(cmd)
  File "/usr/lib/python3.8/subprocess.py", line 359, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib/python3.8/subprocess.py", line 340, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.8/subprocess.py", line 858, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.8/subprocess.py", line 1704, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
OSError: [Errno 7] Argument list too long: 'leader-set'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-vault-1/charm/reactive/vault_handlers.py", line 1105, in create_certs
    vault_pki.update_cert_cache(request,
  File "/var/lib/juju/agents/unit-vault-1/charm/lib/charm/vault_pki.py", line 479, in update_cert_cache
    hookenv.leader_set({PKI_CACHE_KEY: json.dumps(pki_cache)})
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charmhelpers/core/hookenv.py", line 1182, in inner_translate_exc2
    raise to_exc
NotImplementedError

Key part:

OSError: [Errno 7] Argument list too long: 'leader-set'

This occurred due to this code:

    hookenv.log('Saving certificate for "{}" '
                '(cn: "{}") into cache.'.format(request.unit_name,
                                                request.common_name),
                hookenv.DEBUG)
    pki_cache[request.unit_name] = unit_cache
    hookenv.leader_set({PKI_CACHE_KEY: json.dumps(pki_cache)})

i.e. with each new certificate, pki_cache[] gets bigger, and thus the CLI leader-set command gets bigger than the CLI command line can handle (e.g. Argument list too long).

Suggested solution:

have a key for each unit with PKI_CACHE_KEY as a prefix, and mangle the unit_name so that it is an acceptable key.