Fix a couple of problems related to data used by the phases runners
of `init` and `join`.

1) make `newInitData()` and `newJoinData()` return pointers.

Methods of the data objects returned by these functions should
be able to modify fields in the data objects - e.g.
`func (d initData) Client()`. This allows us to store a state and
not execute the same logic multiple times - e.g. obtaining a client.

A side effect of this change is that the `new...` functions must return
pointers, so that casting the data object in a phase, from
`workflow.RunData` to a locally defined interface, works.

2) Make it possible to pass arguments from a parent command
to a sub-phase with regards to data initialization.