Remove ZK paths suport names in vtctl commands

1. ZK path is resolved in methods tabletParamToTabletAlias,
   shardParamToKeyspaceShard and keyspaceParamsToKeyspace.
   Therefore, all of them are refactored to not accept ZK path
   and return error if do so.
2. Move vtPathToCell and keyspaceParamToKeyspace to
   vt/vtctl/plugin_zktopo.go since they contains zk
   logic only.
3. Remove resolveWildcards method in vtctl.go
4. Remove most zk stuff in command description