Bound concurrent syscalls; avoid I/O starvation EUCA-3032

- Add system.exec.restricted_concurrent_ops property
  (default: 2)
- Add system.exec.max_restricted_concurrent_ops property
  (default: 'dd,tar,gunzip')
- Restrict number of concurrent calls to anything in the list
  system.exec.restricted_concurrent_ops to no more than
  system.exec.max_restricted_concurrent_ops concurrent executions
Merge branch 'dev/grze/EUCA-3032' into testing