etcdserver: stop using addScheme

This standardizes the flags to use a list of URLs everywhere. The next step
is to enforce the scheme based on TLS settings and support compat flags.
Since you aren't ignoring any elements of split string, you could pass a nonzero length to the `make` call and assign directly to indices if you do this. It's definitely more efficient to preallocate the array, but I'm not sure if it will make the code less readable.