ceph-defaults: set is_atomic variable

ceph-defaults: set is_atomic variable

This variable is needed for containerized clusters and is required for
the ceph-docker-common role. Typically the is_atomic variable is set in
site-docker.yml.sample though so if ceph-docker-common is used outside
of that playbook it needs set in another way. Moving the creation of
the variable inside this role means playbooks don't need to worry
about setting it.

fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1558252

Signed-off-by: Andrew Schoen <aschoen@redhat.com>
(cherry picked from commit 6cffbd5409353fc1ce05b3a4a6246d6ef244e731)