D-Bus API for storage issues

[joseivanlopez]

Problem

The Agama D-Bus API has the validation concept, which is provided by the org.opensuse.Agama1.Validation interface. That validation is used by clients to check if there is any error. But a more generic mechanishm is required to report diffirent kind of issues, for example warnings. This would be useful, for example, to get the list of issues reported by the storage probing.

Solution

Add a new org.opensuse.Agama1.Issues interface. That interface would replace the previous org.opensuse.Agama1.Validation interface. Now, services can report different types of issues (warn and error for now). Clients can subscribe to changes in the issues.

This PR only adapts the storage service to use the issues interface instead of validations. The rest of services will be adapted later in a serparate PR.

• See https://gist.github.com/joseivanlopez/89c41b19335bbf72539ae468e72d5507.

Note: Failures in the integration tests are expected. They will be fixed after adapting the web UI, see #540.

Testing

• Added a new unit tests
• Tested manually

[coveralls]

Pull Request Test Coverage Report for Build 4817478639

• 124 of 142 (87.32%) changed or added relevant lines in 11 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 74.043%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
service/lib/agama/dbus/interfaces/issues.rb	20	21	95.24%
service/lib/agama/storage/proposal.rb	11	12	91.67%
service/lib/agama/dbus.rb	0	3	0.0%
service/lib/agama/dbus/storage/manager.rb	9	12	75.0%
service/lib/agama/dbus/interfaces.rb	0	10	0.0%

Files with Coverage Reduction	New Missed Lines	%
service/lib/agama/dbus.rb	1	0%
service/lib/agama/storage/proposal.rb	1	93.88%

Totals
Change from base Build 4796705073:	0.2%
Covered Lines:	4848
Relevant Lines:	6318

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💛 - Coveralls

[dgdavid]

LGTM, but I'd wait for another review from someone with more D-Bus background.