all: backward compatibility between stable-2.2 and 3.0

all: backward compatibility between stable-2.2 and 3.0

stable-3.0 brought numerous changes in ceph-ansible variables, this PR
aims to maintain backward compatibility for someone running stable-2.2
upgrading to stable-3.0 but keeps its groups_vars untouched.
We will then determine the right options to make sure the upgrade works
but we are expecting that new variables should be used.

We will drop this in a near future, maybe 3.1 or 3.2.

Signed-off-by: Sébastien Han <seb@redhat.com>