cgroups: Add MaxRAMPercent and MaxSwapPercent config parameters

As a failsafe we may want to put a hard limit on memory.limit_in_bytes
and memory.memsw.limit_in_bytes when using cgroups. This patch adds
MaxRAMPercent and MaxSwapPercent which are taken as percentages of
available RAM (RealMemory as reported by slurmd), and which will be
applied as upper bounds when creating memory controller cgroups.