tracking ER to add and remove leaves from L3out

[ddastoli]

this issue is to track the capability of the tool to allow adding and removing leaf switches from L3out after the initial execution of the tool.
ideally if a cluster exists, we should not load the 'existing_cluster' but rather give option to user to delete or modify existing cluster. If user wants to modify, then he/she will be prompted with the option to change the leaves configured for the L3out.