scripts/check_site_lib.lua: introduce in_site() and in_domain() helpers

These helpers are no-ops for now, until the actual multi-domain support is
added.

scripts/check_site_lib.lua

+function in_site(var)
+   return var
+end
+
+function in_domain(var)
+   return var
+end
+
+
 local function loadvar(varname)
    local ok, val = pcall(assert(loadstring('return site.' .. varname)))
    if ok then