[OHAI-439] Add OpenVZ plugin to parse data from /proc/user_beancounters

http://tickets.opscode.com/browse/OHAI-439

output looks like


```
root@block852638-iap:~# ohai openvz
[2013-01-28T22:52:41+00:00] WARN: [inet] no ip on venet0
[2013-01-28T22:52:41+00:00] WARN: unable to detect ipaddress
[2013-01-28T22:52:41+00:00] WARN: unable to detect macaddress
{
  "othersockbuf": {
    "maxheld": 65760,
    "failcnt": 0,
    "barrier": 4194304,
    "limit": 8388608,
    "held": 25432
  },
  "oomguarpages": {
    "maxheld": 82836,
    "failcnt": 0,
    "barrier": 262144,
    "limit": 9223372036854775807,
    "held": 80387
  },
  "numothersock": {
    "maxheld": 105,
    "failcnt": 0,
    "barrier": 1024,
    "limit": 1024,
    "held": 95
  },
  "numsiginfo": {
    "maxheld": 69,
    "failcnt": 0,
    "barrier": 512,
    "limit": 512,
    "held": 0
  },
  "physpages": {
    "maxheld": 216508,
    "failcnt": 0,
    "barrier": 262144,
    "limit": 9223372036854775807,
    "held": 103851
  },
  "lockedpages": {
    "maxheld": 0,
    "failcnt": 0,
    "barrier": 64,
    "limit": 64,
    "held": 0
  },
  "memory": {
    "total": 1048576
  },
  "tcprcvbuf": {
    "maxheld": 7541904,
    "failcnt": 0,
    "barrier": 10485760,
    "limit": 15728640,
    "held": 81920
  },
  "numtcpsock": {
    "maxheld": 7,
    "failcnt": 0,
    "barrier": 5120,
    "limit": 5120,
    "held": 5
  },
  "kmemsize": {
    "maxheld": 21643264,
    "failcnt": 0,
    "barrier": 67108864,
    "limit": 75497472,
    "held": 18798700
  },
  "dgramrcvbuf": {
    "maxheld": 2312,
    "failcnt": 0,
    "barrier": 1048576,
    "limit": 1048576,
    "held": 0
  },
  "tcpsndbuf": {
    "maxheld": 464472,
    "failcnt": 0,
    "barrier": 10485760,
    "limit": 15728640,
    "held": 87200
  },
  "vmguarpages": {
    "maxheld": 0,
    "failcnt": 0,
    "barrier": 393216,
    "limit": 9223372036854775807,
    "held": 0
  },
  "shmpages": {
    "maxheld": 1005,
    "failcnt": 0,
    "barrier": 262144,
    "limit": 262144,
    "held": 653
  },
  "numiptent": {
    "maxheld": 20,
    "failcnt": 0,
    "barrier": 2048,
    "limit": 2048,
    "held": 20
  },
  "numpty": {
    "maxheld": 3,
    "failcnt": 0,
    "barrier": 64,
    "limit": 64,
    "held": 1
  },
  "numflock": {
    "maxheld": 7,
    "failcnt": 0,
    "barrier": 1024,
    "limit": 1024,
    "held": 1
  },
  "numproc": {
    "maxheld": 168,
    "failcnt": 0,
    "barrier": 1024,
    "limit": 1024,
    "held": 104
  },
  "privvmpages": {
    "maxheld": 260691,
    "failcnt": 207,
    "barrier": 393216,
    "limit": 442368,
    "held": 241239
  },
  "numfile": {
    "maxheld": 520,
    "failcnt": 0,
    "barrier": 10240,
    "limit": 10240,
    "held": 322
  },
  "dcachesize": {
    "maxheld": 9437184,
    "failcnt": 0,
    "barrier": 8388608,
    "limit": 9437184,
    "held": 9407477
  }
}
```

Take special note of `ohai openvz/memory/total`