lua: implement stubs for etcd client

To enable etcd client set the following CMake variables:

 - ENABLE_ETCD_CLIENT: ON
 - ETCD_CLIENT_DIR: path to a directory with etcd client Lua source
   files

If ENABLE_ETCD_CLIENT is unset, then a stub implementation is built,
which spits an error on attempt require etcd-client.

Needed for https://github.com/tarantool/tarantool-ee/issues/443

MSG="stubs for enterprise version, no user-visible changes"

NO_CHANGELOG=MSG
NO_DOC=MSG
NO_TEST=MSG