NXCALS-1172 Enforces fixed ansible version on Jenkins slave machines (openstack)

#Merge request template @acc-logging-team - please have a look for approval.

We have an issue (since the pipeline was unstacked), with the expected execution of Jenkins staging dsl pipeline. In more details, ansible was failing to run playbooks that target the localhost interface of the machine.

Aparently there's an issue that affects our installed version of ansible, (it was 2.4.x on jenkins slaves). The issue is described here

This MR downgrates the version to 2.3.x where the issue is not observed. Until further notice, we can stay with this one.