Document the etcd and other configuration cleanup steps

[atinmu]

I was trying to spend some time in (re)setting up a GD2 cluster of 3 nodes in my docker containers and peer probe failed 3/3 times randomly. I suspect this has something to do with a uncleaned etcd clustering data. Having some pointers in the wiki about how to start from the scratch if there's already left over data of the old cluster. Even a cleanup script in the repo might be very helpful.

@Madhu-1 @prashanthpai @aravindavk @kshlm Thoughts?

[rishubhjain]

@atinmu Already a PR in place, will address the review comments and will make sure already known issues are documented.

[atinmu]

@rishubhjain ping! this is pending since long time.