[consfigurator] Add pass data source

[David Bremner]

[PATCH consfigurator 1/3] create package consfigurator.data.util

The first two patches are refectoring so that I can use some of the existing
code in patch 3/3. I added a new package "inside" consfigurator.data to hold
the reused code.

[PATCH consfigurator 2/3] factor out gpg-file-as-string

I did push the handler-case inside the outermost function call. I _think_ it's
OK, but you'd know better than I.

[PATCH consfigurator 3/3] introduce the pass data store.

In the end I decided it was not a great bother just to specifify
"--user-passwd-foo" instead of "foo" in client code. It also avoids some
collisions, although I don't know how likely those are.