Fix return value of rebar:overlay/2
Fix return value of rebar:overlay/2

When 'rebar overlay' is run rebar_core got a wrong result
{Config, ok} from rebar_reltool instead of {ok, Config}.

oubiwann/rebar,rebar/rebar,jacktang/rebar,macintux/rebar,bsmr-erlang/rebar,evnix/rebar,GabrielNicolasAvellaneda/rebar,zugolosian/rebar3,matwey/rebar,blt/rebar3,GabrielNicolasAvellaneda/rebar,heroku/rebar,thomasbhatia/rebar,linearregression/rebar3,platinumthinker/rebar,lrascao/rebar,matwey/rebar,surik/rebar3,K2InformaticsGmbH/rebar,matwey/rebar,deadok22/rebar,apache/couchdb-rebar,wcy123/rebar,cloudant/couchdb-rebar,syl20bnr/rebar,project-fifo/rebar3,cloudant/couchdb-rebar,fauxsoup/rebar3,beni55/rebar-1,cloudant/couchdb-rebar,basho/rebar,zugolosian/rebar3,heroku/rebar,beni55/rebar,lpgauth/rebar,SiDen/rebar,rebar/rebar,benoitc/rebar3,saleyn/rebar3,lpgauth/rebar,csyangchen/rebar,oubiwann/rebar,shkumagai/rebar,scatterbrain/rebar,lpgauth/rebar,shkumagai/rebar,kellymclaughlin/rebar3,paulo-ferraz-oliveira/rebar,sile/rebar3,jacktang/rebar,jbalint/rebar3,scatterbrain/rebar,thomasbhatia/rebar,choptastic/rebar,lrascao/rebar,matwey/rebar,K2InformaticsGmbH/rebar,linearregression/rebar3,surik/rebar3,macintux/rebar,SiDen/rebar,lrascao/rebar,paulo-ferraz-oliveira/rebar,paulo-ferraz-oliveira/rebar,deadok22/rebar,CMoH/rebar,oubiwann/rebar,macintux/rebar,K2InformaticsGmbH/rebar,wcy123/rebar,lrascao/rebar,thomasbhatia/rebar,bsmr-erlang/rebar,lrascao/rebar3,processone/rebar3,platinumthinker/rebar,fauxsoup/rebar3,norton/rebar3,shkumagai/rebar,platinumthinker/rebar,csyangchen/rebar,dcarastan/rebar,syl20bnr/rebar,lrascao/rebar,RubberCthulhu/rebar,blt/rebar3,jacktang/rebar,walrusVision/rebar3,vladdu/rebar3,rebar/rebar3,SiDen/rebar,basho/rebar,fabiankrol/rebar3,walrusVision/rebar3,dcarastan/rebar,apache/couchdb-rebar,rebar/rebar3,norton/rebar,erlang/rebar3,norton/rebar,CMoH/rebar,choptastic/rebar,benoitc/rebar3,beni55/rebar,norton/rebar,jbalint/rebar3,evnix/rebar,beni55/rebar-1,matwey/rebar,rebar/rebar,djnym/rebar3,ferd/rebar3,scatterbrain/rebar,K2InformaticsGmbH/rebar,csyangchen/rebar,beni55/rebar,dcarastan/rebar,syl20bnr/rebar,bsmr-erlang/rebar,CMoH/rebar,RubberCthulhu/rebar,tsloughter/rebar3,sile/rebar3,processone/rebar3,GabrielNicolasAvellaneda/rebar,bsmr-erlang/rebar,norton/rebar,fishcakez/rebar3,K2InformaticsGmbH/rebar,fabiankrol/rebar3,project-fifo/rebar3,apache/couchdb-rebar,evnix/rebar,choptastic/rebar