Re-struct DeferredEventMixin class

The way that the custom_assess_status_check is declared in DeferredEventMixin prevents it from being defined elsewhere and this may cause conflicts in future implementations and features.

Having DeferredEventMixin separated from the BaseOVNChassisCharm has the benefit of allowing the deferred event functionality to be added or removed for each Openstack release, but it might not be optimal having methods of the class OpenStackCharm outside BaseOVNChassisCharm that are inherit just in the OpenStack release (TrainOVNChassisCharm and UssuriOVNChassisCharm).

I see two possible ways of solving this:

- Inherit DeferredEventMixin inside BaseOVNChassisCharm and declare the method custom_assess_status_check inside BaseOVNChassisCharm;

- Rename the method and have a new custom_assess_status_check for each release that would call this renamed method and others that might be needed.

Thanks!