riemann-dash: Use a 100% max for df metrics

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Change the logging comment

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Do a threshold-check by default

Instead of adding the threshold check as the last leaf of every branch,
add it once for the whole index.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Create load & memory summaries

Summarise load and memory across all hosts, and put it on the Services
tab on the dashboard.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>