tests: remove CEPH_STABLE environment variable

tests: remove CEPH_STABLE environment variable

I'm removing this because when we use an 'rhcs' scenario then we attempt
to set CEPH_STABLE=false as an environment variable. The issue with that
is because the value is coming from an environment variable it is always
treated as a string and ansible treats that as a boolean True. I plan to
set the ceph_stable value with our rhcs_setup.yml playbook instead of
relying on ---extra-vars and environment variables.

Related ansible issue: https://github.com/ansible/ansible/issues/17193

Signed-off-by: Andrew Schoen <aschoen@redhat.com>
(cherry picked from commit 63983fa0b728571518ecdeca249ef37506a00a64)