Assert etcd:list by "/"

[vanyarock01]

If you set an empty etcd prefix / in conf.lua, e.g:

etcd = {
    endpoints = {'http://etcd:2379'},
    prefix = '/',
}

you will get an error like this:

instance      | LuajitError: /instance/.rocks/share/tarantool/config/etcd.lua:148: bad argument #1 to 'sub' (string expected, got nil)

I think we can write an assert for this case.

Logs after:

instance      | LuajitError: /pairdb/.rocks/share/tarantool/config/etcd.lua:194: Failed to list keys by "/", node.key is nil

[ochaton]

seems legit

config aims to run tarantool applications (clusters, shards or proxies). normally we need to point user to write proper configuration in ETCD. @vanyarock01 will you update README.md with example of usage for developer and yaml structure of config for devops?