[INFRA-84] adding vnstat check to nagios

This script uses vnstat output to check the daily total bandwidth utilization of the day before. Looking at the hourly utilization output (vnstat -h), there's a great degree of variability in the utilization, so to avoid false alarms we need to at least go to daily.

OTOH, if the time span of measurement is too long, then we'd take that much time to notice the problem.  daily or weekly is a good compromise.

See the sample data output below:

$ vnstat -d

 eth0  /  daily

         day         rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
      05/04/14     11.61 GiB |  151.50 GiB |  163.11 GiB |   15.84 Mbit/s
      05/05/14     18.37 GiB |  167.82 GiB |  186.18 GiB |   18.08 Mbit/s
      05/06/14     12.54 GiB |  176.53 GiB |  189.07 GiB |   18.36 Mbit/s
      05/07/14     13.07 GiB |  169.65 GiB |  182.73 GiB |   17.74 Mbit/s
      05/08/14     12.49 GiB |  152.46 GiB |  164.95 GiB |   16.01 Mbit/s
      05/09/14     13.54 GiB |  167.91 GiB |  181.45 GiB |   17.62 Mbit/s
      05/10/14     10.72 GiB |  149.50 GiB |  160.22 GiB |   15.56 Mbit/s
      05/11/14     15.25 GiB |  141.75 GiB |  157.01 GiB |   15.24 Mbit/s
      05/12/14     16.06 GiB |  168.05 GiB |  184.11 GiB |   17.88 Mbit/s
      05/13/14     13.25 GiB |  144.43 GiB |  157.68 GiB |   15.31 Mbit/s
      05/14/14     18.24 GiB |  160.37 GiB |  178.61 GiB |   17.34 Mbit/s
      05/15/14     14.30 GiB |  154.11 GiB |  168.41 GiB |   16.35 Mbit/s
      05/16/14     12.99 GiB |  153.21 GiB |  166.20 GiB |   16.14 Mbit/s
      05/17/14      9.87 GiB |  127.64 GiB |  137.51 GiB |   13.35 Mbit/s
      05/18/14     11.43 GiB |  186.48 GiB |  197.91 GiB |   19.22 Mbit/s
      05/19/14     14.08 GiB |  171.26 GiB |  185.35 GiB |   18.00 Mbit/s
      05/20/14     13.47 GiB |  149.67 GiB |  163.14 GiB |   15.84 Mbit/s
      05/21/14     12.67 GiB |  150.21 GiB |  162.89 GiB |   15.81 Mbit/s
      05/22/14     13.43 GiB |  168.17 GiB |  181.61 GiB |   17.63 Mbit/s
      05/23/14     14.06 GiB |  165.47 GiB |  179.53 GiB |   17.43 Mbit/s
      05/24/14      9.75 GiB |  132.61 GiB |  142.36 GiB |   13.82 Mbit/s
      05/25/14     10.10 GiB |  131.50 GiB |  141.60 GiB |   13.75 Mbit/s
      05/26/14     13.81 GiB |  180.48 GiB |  194.28 GiB |   18.86 Mbit/s
      05/27/14     14.51 GiB |  187.94 GiB |  202.45 GiB |   19.66 Mbit/s
      05/28/14     12.76 GiB |  163.84 GiB |  176.60 GiB |   17.15 Mbit/s
      05/29/14     11.95 GiB |  157.68 GiB |  169.63 GiB |   16.47 Mbit/s
      05/30/14     11.62 GiB |  142.46 GiB |  154.08 GiB |   14.96 Mbit/s
      05/31/14      9.97 GiB |  136.38 GiB |  146.35 GiB |   14.21 Mbit/s
      06/01/14     11.49 GiB |  142.08 GiB |  153.56 GiB |   14.91 Mbit/s
      06/02/14     12.43 GiB |  140.95 GiB |  153.38 GiB |   16.51 Mbit/s
     ------------------------+-------------+-------------+---------------
     estimated     13.79 GiB |  156.37 GiB |  170.16 GiB |
$ vnstat -h
 eth0                                                                     21:48
  ^      t
  |      t
  |      t
  |      t
  |      t                       t                                t
  |      t  t                    t                                t
  |      t  t                    t                                t
  |      t  t  t  t  t  t  t     t  t  t  t  t                    t
  |   t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t
  |   t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t
 -+--------------------------------------------------------------------------->
  |  22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

 h  rx (KiB)   tx (KiB)      h  rx (KiB)   tx (KiB)      h  rx (KiB)   tx (KiB)
22     400518    5232033    06     482903    5788799    14     442731    5596439
23     757844   20282082    07     705398   13406975    15     682697    5755081
00     660549   12053164    08     546927    6238720    16     590482    5903267
01     879250    7792739    09     680388    6849908    17     539212    5202574
02     543242    6908622    10     537380    6116265    18     622976   12975046
03     579962    7276801    11     923817    6939105    19     457193    5220198
04     640700    6850051    12     688261    6043098    20     353300    1351220
05     671866    6948858    13     513767    5551320    21     352394    1315830