Add idempotent converge method in resource class

In convergence architecture the resources are monitored to check if they are indeed in the desired state (as given in template). This monitoring can be manually done using stack-check API or automatically by a continuous observer. When there is a divergence in resource state and properties, the resource need to be converged again.

A user or continuous observer detects a divergence and it needs a way to converge back the resource to desired state. This spec is proposal to add an idempotent "converge" method on resource which will take the latest resource definition from template and check with reality for current change in state of affairs and will update the resource accordingly.

The converge method will decide on which CRUD operation to be done on resource. The get_reality already
gets the changes from reality and converge method can make use of it.

The converge method needs to be idempotent so that the continuous observer or manual observation
doesn't need to send the changes (the converge method doesn't have to depend on it) and they
should be able to invoke converge without having to worry about how much or what has changed.