cgroups: Add new config parameter MinRAMSpace

Add a new configuration parameter MinRAMSpace which sets a lower bound on
memory.limit_in_bytes and memory.memsw.limit_in_bytes . This is required in
case an administrator or user sets an absurdly low value for memory limit,
potentially causing the slurmstepd to be terminated by the OOM killer.

MinRAMSpace is set in MB of RAM and is 30 by default. (An arbitrarily
chosen value)