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