Migrate test_dashboard_basic_ops to tempest clients

Just switched from OfficialClientTest to ScenarioTest because
TestDashboardBasicOps does not use methods of OfficialClientTest.

Change-Id: I3b8eb90e058cde03cb40f120bb2c268f31ae078c
Partially-implements: bp:tempest-client-scenarios
Merge "Migrate test_dashboard_basic_ops to tempest clients"