The manager option in cells group is deprecated in mitaka
and hence removed in newton.
Checked cells.py for check_deprecation_status and
removed the tag according to wiki.
https://wiki.openstack.org/wiki/ConfigOptionsConsistency
Blueprint centralize-config-options-newton
Change-Id: Iebd6c261d7e5ca531dbf473f1d986b2e42f02e41