Jenkins machine total RAM should not be hardcoded

[kennknowles]

In PR#5218, we update Jenkins Gradle config to dynamically set per-worker memory usage based on total RAM and number of worker processes. However, the OperatingSystemMXBeam.getTotalPhysicalMemory() API is for some reason returning the wrong value. For now, the total memory value is hard-coded until we can find a better alternative.

Imported from Jira BEAM-4230. Original Jira may contain additional context.
Reported by: swegner.