Extract JSON validation code into SystemDescriptionValidator

The validation code in SystemDescription grew enough to warrant a new
class -- SystemDescriptionValidator. In this commit, all JSON validation
code is moved there and tests are adapted.

Note I didn't attempt to do any cleanups in the code nor in the tests
(beside removing some now useless "describe" blocks). This will come
later. As a result, this is mostly a mechanical change.
Extract file data validation code into SystemDescriptionValidator

As a follow-up to JSON validation extraction, the file data validation
code is extracted into SystemDescriptionValidator.

I didn't attempt to do any cleanups in the code nor in the tests. This
will come later. As a result, this is mostly a mechanical change.
Kill system_description_validation_spec.rb

Tests in system_description_validation_spec.rb were originally separated
mostly for clarity reasons. Now that we have a dedicated validation
class (SystemDescriptionValidator), let's move them into its tests and
delete the file.
Merge pull request #194 from SUSE/extract-system-description-json-validator

[Review] Request from 'dmajda' @ 'SUSE/machinery/extract-system-description-json-validator'