Fix sub dir 'rebar generate' regression (#253)

If the dir we're about to process contains "reltool.config" and the
command to be applied is "generate", then it's safe to process. We
do this to retain the previous behavior of specifying {sub_dirs, ["rel"]}
and have "rebar generate" invoked from base_dir generate a release
in rel/. Without this workaround you'd have to run "rebar -r generate"
(which you don't want to do if you have deps or other sub_dirs)
or "cd rel && rebar generate".