Fix dependency cycle between osquery_core and plugins_config_parsers

[Smjert]

The osquery_core library depends on plugins_config_parsers in the addLegacyFieldsAndDecorations, because it has to access the flag FLAGS_decorations_top_level, which is defined in plugins_config_parsers.
Vice versa plugins_config_parsers (as most of the rest of the code in osquery) depends on osquery_core.

A first hacky fix could be to not use the FLAGS_decorations_top_level variable to access that information (which is also not thread safe!), but use the gflag functions to get the flag by name instead.