WL#13773: Increase default group_replication_member_expel_timeout

EXECUTIVE SUMMARY
=================

Change the default group_replication_member_expel_timeout from 0 to 5 seconds.

Currently, group_replication_member_expel_timeout is set to 0 seconds, which
means that a member will expel immediately another group member which is
suspected of leaving the group. Since there is a 5 second waiting period  until
creating the suspicion that the member has left the group, the member is
actually expelled 5 seconds after becoming unreachable. If the expelled member
is the primary a new primary is elected automatically.

Set the default group_replication_member_expel_timeout to 5 seconds, in order to
decrease the likelihood of unnecessary expulsions and primary failovers on
slower networks or in the presence of transient network failures, following
feedback gathered from customers.

The new value for the default implies that the member will be expelled 10
seconds after becoming unreachable.

FUNCTIONAL REQUIREMENTS
=======================

FR1: The default value of the variable group_replication_member_expel_timeout
shall be set to 5.

Change the default value of group_replication_member_expel_timeout from '0' to
'5' seconds.

Run MTR and JET regression test suites with the new default value.

Evaluate the list of MTR and JET tests failing. Check if these are expected due
to the change in group_replication_member_expel_timeout and set it to zero for
such test cases.