add max-connect-errors config parameter

[Alvaro Uria]

Per https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_max_connect_errors

* max_connect_errors can have a default value of 10 or 100.
* number of OpenStack API workers can be high if the number of vcpus is, too (and worker-multiplier value is set to the default, 2.0)

We've seen in a couple of environments that MySQL blocks connections from a host after a config change on such host, via Juju. When such re-configuration happens, OpenStack API services restart and the servers try to reconnect to MySQL.

"mysqladmin flush host" is the only action that will resolve the situation.

* Request:
Be able to tweak max_connect_errors value. If change is done when the MySQL service is clustered and running, "set global max_connect_errors=$VALUE" should be sent to each server (apart from applying the configuration on the cnf file; no need to restart MySQL).

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-percona-cluster/+/806237

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-percona-cluster/+/806237
Committed: https://opendev.org/openstack/charm-percona-cluster/commit/75fcf19f33dff690565d24ab06226d908ae0ef08
Submitter: "Zuul (22348)"
Branch: master

commit 75fcf19f33dff690565d24ab06226d908ae0ef08
Author: Nicholas Njihia <email address hidden>
Date: Thu Aug 26 17:18:09 2021 +0300

    Add max_connect_errors configuration option.

    This change adds the config option max_connect_errors for MySQL to this
    charm and sets a default of 100.
    Commit also includes the inclusion of this (default) config setting in the
    unit tests.
    Closes-Bug: #1776908

    Change-Id: I33b9e29bd64ad8a1fec0edc3dfd657a87648b537