Setting new ssh_key and after that new ssh_cert results in apache2 not running

The issue was discovered while working on a reconfiguration of aodh units, but possibly is in a shared layer and affects multiple openstack charms.

While renewing ssl certificate and key via charm config I observed the following behavior:

1. Update ssl_key:
   juju config aodh ssl_key="$(base64 new_ssl_key)"

2. Apache2 stops working as at the moment the old certificate and the new key don't match

3. Update ssl_cert:
   juju config aodh ssl_cert="$(base64 new_ssl_cert)"

4. Now ssl key and cert match but charm tries to reload apache2 which can't work given apache2 is not running because of the previous mismatch.

Workaround is to start apache2:
   juju run -a aodh "systemctl start apache2"

At step 4, charm may check if apache2 is not running and try to start it instead of reloading.
Same should happen if a new cert is provided before setting the new key.

The issue can be avoided setting ssl_key and cert at the same time:
   juju config aodh ssl_key="$(base64 new_ssl_key)" ssl_cert="$(base64 new_ssl_cert)"