Timescale for Perf dashboards needs to be configurable

Yesterday, we were trying to determine if the golang upgrade had adversely affected performance. The Perf dashboard was not in any way useful for this, which was disappointing and effectively led to a 1-day delay in releasing the beta.

The primary problem is that the timescale for the perf dash is not configurable, so it's not possible to get useful information out of it to examine specific changes. For example, look at gce-100Nodes, E2E, DensityPodStartup. The lines are so dense that you can't read anything at all.

We should be able to choose start and end dates for the graph data, which would allow (for example) looking at the most recent 24 hours of 100nodes.

Start by locating the Perf dashboard implementation and checking how gce-100Nodes, E2E, and DensityPodStartup currently select graph data. Add a way to choose start and end dates, then verify that a focused range such as the most recent 24 hours produces readable dashboard data.

Written by the indexing model from the issue text.