A fix and a test case for Bug#928413 "Lua malfunctions..."

We didn't check whether or not Lua bindings executed correctly,
so we didn't pay attention to these problems:

luaL_Buffer was misused, and when configuration
would exceed 4K allocated on stack, this led
to lost pieces of Lua script prepared by
tarantool_lua_load_cfg().

box.on_reload_configuration() did not work correctly with
disabled spaces.

Fix the above mentioned issued and add a test.