Popover: add option to trigger a popover on hover

[mcarrano]

Want to optionally support triggering a popover on hover. Triggering on-click is still the default behavior. See this design issue for more background: https://github.com/patternfly/patternfly-design/issues/1131

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.