Add test cases to reproduce the issue

where tag fields are not available in the service report.

https://bugzilla.redhat.com/show_bug.cgi?id=1127779
Add Service model to MiqExpression::TAG_CLASSES.

So the tags fields are listed in the service report.

https://bugzilla.redhat.com/show_bug.cgi?id=1127779
Merge pull request #435 from lfu/tag_fields_service_report_1127779

Add Service model to MiqExpression::TAG_CLASSES