Login to dashboard

This ensures that home_page_logged_in always lands you on the Dashboard Page, even if the user's start page has been changed.

@mfalesni's work from https://github.com/RedHatQE/cfme_pages/pull/242 was orphaned in the cfme_pages -> cfme_tests repository merge, so I brought it over here and applied my review comments to make sure the work isn't lost.