Add callback function to inform user if memory/cpu config set.

[praveenkumar]

As of now we are not letting users know that if they set memory or cpu configs as part of config set they need to start a fresh instance to apply those configs.

Something we use to have in minishift side.

$  ./minishift config set memory 4096
No Minishift instance exists. New 'memory' setting will be applied on next 'minishift start'

$ ./minishift config set memory 4096
You currently have an existing Minishift instance. Changes to the 'memory' setting are only applied when a new Minishift instance is created.
To let the configuration changes take effect, you must delete the current instance with 'minishift delete' and then start a new one with 'minishift start'.