dnsdist: Disable Lua configuration directives in YAML mode

dnsdist: Disable Lua configuration directives in YAML mode

This commit restricts the Lua directives available to a Lua script
loaded alongside with a YAML configuration, disabling all Lua
configuration directives. This prevents a mess when settings are
set to different values in YAML and Lua, and make it clear that the
when a YAML configuration is used, any Lua file should only be used
to provide functions to the YAML configuration.
This behaviour can be reverted via the `enableLuaConfiguration`
directive if really needed.