ceph-{ansible,installer}-rpm: fix mock group addition

ceph-{ansible,installer}-rpm: fix mock group addition

When an ephemeral slave is first created, we have to add the
jenkins-build UID to the "mock" group so that it can run mock.

The problem is that "usermod -a -G" will not affect the current login
shell. When the ceph-*-rpm jobs run "usermod" for the first time,
the jenkins-build UID obtains those group rights for any future jobs,
but not the current job.

The result is that every time an ephemeral slave boots, it will fail its
first ceph-*-rpm build job (and future ones succeed).

Call "newgrp" to refresh the groups for this login shell, so that all
mock operations succeed, including the first one on each slave.