Merging defaults and per-release config incorrect

[tsloughter]

@Taure pointed out in rebar3 slack that the config:

{relx, [{release, {myapp, "0.1.0"},
         [myapp],
         [{dev_mode, false},
          {include_erts, true},
          {extended_start_script, true}]}
       ]}.

does not work correctly, ERTS is not included in the tarball.

A config with include_erts in the top level of the relx config, like:

{relx, [{release, {myapp, "0.1.0"},
         [myapp]},
         {dev_mode, false},
         {include_erts, true},
         {extended_start_script, true}
       ]}.

does work.

So something is wrong with merging defaults (include erts is false by default) and the values set in the per-release list of configuration values -- or it is never reading those in some cases.