[Accessibility] Drop downs no not behave like navigation lists in the screen reader

[aphelionz]

Steps to reproduce (assumes ChromeVox or similar)

1. Navigate to Infrastructure
2. Tab through to any of the main drop downs (Metric, Group By)
3. Press enter

Actual Result

4. Elements are accessible but lacking context of typical dropdowns

Expected Result

5. Screen reader should read sometihng like "Drop down, 5 items, item 1, CPU Usage"

Meta Issue #26854
Kibana Version: 6.5
Relevant WCAG Criteria: 3.2.3 Consistent Navigation

[elasticmachine]

Pinging @elastic/infrastructure-ui