Add cpu usage to regression benchmarks (3rd attempt)
Add cpu usage to regression benchmarks (3rd attempt)

Summary:
Tested by running rocks/tools/debug_regression_test.sh and verifying the local output:
```
 cat local/rocks_regression_tests/OPTIONS-viewstate-66-1262-5000/2017-04-13-11-34-51/SUMMARY.csv
                               commit id,                benchmark,                     user@host,num-dbs,key-range,key-size,value-size,compress-rate,ops-per-thread,num-threads,  cache-size,flushes,compactions,ops-per-s,       p50,       p75,       p99,     p99.9,    p99.99,debug,real-sec,user-sec,sys-sec
d2dce5611ab4b9a671ef518f648adf84d49e5356,               readrandom,                root@localhost,     12,     5000,      66,      1262,           50,           312,         16,  1073741824,      4,         16,   138458,      9380,     11530,     55200,  16803200,  32504000,    1,    0,    0,    0
d2dce5611ab4b9a671ef518f648adf84d49e5356,         readwhilewriting,                root@localhost,     12,     5000,      66,      1262,           50,           312,         16,  1073741824,      4,         16,   104511,
Closes https://github.com/facebook/rocksdb/pull/2157

Differential Revision: D4909238

Pulled By: maysamyabandeh

fbshipit-source-id: dc7bb8569c3c33b9f7c4ba47a757b24d27bb3b31

bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb