:out inheritance of control-flow steps inconsistent or undefined

[flosell]

control-flow steps like in-parallel, run and either inherit the :out text (that's also shown in the UI) but there doesn't seem to be a consistent defined behavior. e.g. in-parallel seems to just return the :out of the last child (whatever order that might be).

Either implement a consistent behavior or don't inherit such attributes at all since they are available from the step itself and are usually not helpful for the next build steps. Other attributes need to be inherited though, e.g. the git-hash that might come out of a wait-for-git step in an either.

[flosell]

Merging all child outputs could be consistent behavior (#124) but is not trivial to do