Resize instance base on workload

In Watcher, administrator can metric the workload of instance and Node, and
watcher can to balance or combine the workload of Node by live migration.

Obviously, the workload of VM is a important feature about user experience. We
will get a good experience and resource optimization if we can adjust the
instance's flavor to the optimum status.

This is the reason why, we should resize instance base on it's workload.

However, watcher and nova can't automatic optimize the instance by resize which
are overload although the cloud environment has free resource.

Use case 1:
As a end user,
I want watcher can to resize the instance to a large flavor when it metric that
instance has a very high workload. In order to Instance can be efficient to deal with overload business.

Use case 2:
As a end user,
I want watcher can to resize the instance to a small flavor when it metric that
instance has a very low workload. In order to Cloud environment use resources more effectively.