etcdhttp: set peer should clean up previous values

We have an init value for peer. When the flag.Parse is called, it will call peer.Set again. Peer.Set should be able to cleanup the previous value.
Referencing the docs:

"Value is the interface to the dynamic value stored in a flag. (The default value is represented as a string.)"
http://golang.org/pkg/flag/#Value

The docs are very clear. I'm wary of putting a panic here since we use peers during the running of the server. I'm not sure what to do.