From version 2.1, ProxySQL has a built-in ProxySQL
Prometheus exporter. This patch adds an option to
easily enable this exporter [1].

[1] https://proxysql.com/documentation/prometheus-exporter

Change-Id: I8776cdc0a6ec9e4e35a2424dd0984488514a711f

By default ProxySQL's default value of max_replication_lag
is 0 which is in fact disabling this feature [1].
If it is greater than 0, ProxySQL will regularly monitor
replication lag and if it goes beyond the configured threshold
it will temporary shun the host until replication catches up.

This should be configurable via kolla-ansible as every
openstack deployment can be different in terms of network
delays, database load etc.. , so user should have option
to configure when database backend will be shunned.

[1] https://proxysql.com/documentation/main-runtime/

Change-Id: I66171638abc712cb84b380042f1d29f54c499e73

Kolla environment currently uses haproxy
to fullfill HA in mariadb. This patch
is switching haproxy to proxysql if enabled.

This patch is also replacing mariadb's user
'haproxy' with user 'monitor'. This replacement
has two reasons:
  - Use better name to "monitor" galera claster
    as there are two services using this user
    (HAProxy, ProxySQL)
  - Set password for monitor user as it's
    always better to use password then not use.
    Previous haproxy user didn't use password
    as it was historically not possible with
    haproxy and mariadb-clustercheck wasn't
    implemented.

Depends-On: https://review.opendev.org/c/openstack/kolla/+/769385
Depends-On: https://review.opendev.org/c/openstack/kolla/+/765781
Depends-On: https://review.opendev.org/c/openstack/kolla/+/850656

Change-Id: I0edae33d982c2e3f3b5f34b3d5ad07a431162844