update: do not enforce `serial: 1` on client nodes

update: do not enforce `serial: 1` on client nodes

There is no need to enforce `serial: 1` on client nodes.
Let's make it parameterizable by introducing a new *extra* variable
`client_update_batch`, if not filled this will default to `{{
ansible_forks }}`.

NOTE: this is only usable as an extra variable passed with
`-e client_update_batch=<num>`

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1650184
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 268f2cef821dcb5835bd925c42585ddda5a07861)