Schemas: Change structure of the "unmanaged_files" schema

Change structure of the "unmanaged_files" schema to eliminate
duplication and make the structure more logical.

Also replace the the '"additionalProperties": false' hack in the
"not_extracted" definition by the "not" keyword. The hack was there to
make the definition mutually-exclusive with "extracted_*" definitions.
(without this, the toplevel "oneOf" keyword wouldn't work). However, it
prevented extending JSON objects representing non-extracted unmanaged
files, which was wrong per our data model design (which says the user
should be able to extend any object with custom properties).
Schemas: Slightly simplify mode regexp in the "unmanaged_files" schema

Slightly simplify the "mode" attribute regexp in schema for the
"unmanaged_files" scope.
Merge pull request #185 from SUSE/schema-unmanaged-files-tweaks

[Review] Request from 'dmajda' @ 'SUSE/machinery/schema-unmanaged-files-tweaks'
Use SystemDescriptionStore to load validation file

Showing the file path requires the SystemDescriptionStore object
to check the real error message.

Removed unnecessary unit test due to other tests validates this.

Moved the validation files to keep uniform direcory layout.

Moved validation tests from system_description_spec to
system_description_validation_spec.
Consolidate integration validation descriptions with unit tests

Moved the integration invalid-json and validation-errors into
subdirectorys to have the same layout as the unit tests.
Add file path to validation error message
Move validate task descriptions to common directory
Adapt error messages and tests to new schema

The config- and changed-managed-files schema was changed in an similar
way as the unmanaged-files inspector so the error messages have
changed.

The regex did not apply anymore so we removed it.
Implement method to clean up json error messages

Implement testable method to clean up the weird json error messages
since they are unusable for some scopes like unmanaged-files.
Use cleanup_json_error_message for json validation error handling