Switch to using a helper method for creating a solo_runner

This is so we can stub the default node attributes that might appear in our omnibus-ctl dna.json attribute file.

As a first candidate, I have moved the package install-dir setting into the dna.json

I logged an issue upstream for an easier way to do this: https://github.com/chefspec/chefspec/issues/861, but because the use of -j and normal attributes is advised against by chef, it seems unlikely chefspec will consider adding the change. (Especially when the approach they recommend, seen in this MR, does work)