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>
Reviewed-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 19cff890eb6083eefdb7b709773313b2c8acbcea)
osd: Fix assert which doesn't apply when compat_mode on

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit edaec9a8361396bd4c12814c16610669694b5b6c)