AutoConfigurator more resilient to unknown physmem

1. Makes figuring out the physicalMemory in the AutoConfigurator more
   resilient to ClassLoader restrictions that would prevent loading the
   interface that has the getTotalPhysicalMemorySize method.
2. Makes AutoConfigurator handle being unable to detect the physical
   memory behave in a better way, by still trusting the heap size and
   not treating it as if the heap has been configured too large.
Merge pull request #2910 from thobe/1.9-auto-config-physmem

[1.9] Make AutoConfigurator more resilient to unknown physical memory size