shrink-mgr: modify existing mgr check
shrink-mgr: modify existing mgr check

Do not rely on the inventory aliases in order to check if the selected
manager to be removed is present.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1967897

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible