Add backward comptible acting set until all OSDs updated

Add configuration variable to override compatible acting set handling.
Later we'll check the osdmap that all OSDs are updated to use new acting sets.

Fixes: #6990

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge pull request #960 from ceph/wip-6990

Add backward comptible acting set until all OSDs updated

Reviewed-by: Samuel Just <sam.just@inktank.com>