Create test descriptions using a helper

Instead of creating test descriptions via SystemDescription#from_json,
let's create them using a helper. This creates a layer between the tests
and SystemDescription#from_json, making things like changing method
parametrs and filling-in uninteresting parameters easier. For example,
we'll be able to omit irrelevant description names and make #from_json's
storage parameter required easily.

Note the change is 100% mechanical -- created descriptions are exactly
as they were before.

Also note I didn't convert #from_json calls where the method itself, or
some code called from it, was the subject of testing.
Don't name test description whose names aren't relevant

The existence of the create_test_description helper allows us not to
explicitly name tests descriptions whose names aren't relevant. This
reduces clutter a bit.
Merge pull request #204 from SUSE/system-description-always-have-store

[Review] Request from 'dmajda' @ 'SUSE/machinery/system-description-always-have-store'