Omnibus Gitaly dashboard should support multiple nodes

During the 2020-05-27 demo, it was observed that the Omnibus Gitaly dashboard does not show more than one Gitaly at a time. Some of the panels, such as the current version, don't even support more than one Gitaly node (should instead be a table). The dashboard only allows viewing Gitalies with job set to "gitaly", while Praefect Gitaly nodes have a job of "praefect-gitaly". This should also be an option when selecting Gitalies.