omnibus dashboard: show usage rather than available

https://gitlab.com/gitlab-org/grafana-dashboards/tree/master/omnibus

Would be better to show memory and disk usage rather than availability so that all the gauges consistently go the same direction.

Discussion started at https://gitlab.com/gitlab-org/gitlab-ce/commit/e91c7469ad0be5f429548d4142ca93c17ec9e71e#note_105695296