Add nfsiostat to checkmk for all the workers nodes

In the same way we should track the nfs server with iostat we need to do the same in all the workers to check the use of NFS we are doing. This will be particularly handy when we move to a distributed filesystem as we will need to know if things are working at the same rate or less.

A sample output of nfsiostat (consider that the output format is kinda broken for now http://linux-nfs.vger.kernel.narkive.com/tjITNl10/patch-nfsiostat-restore-output-format)

nfsiostat 5 100 /var/opt/gitlab/git-data

10.1.0.21:/var/opt/gitlab/git-data mounted on /var/opt/gitlab/git-data:

   op/s		rpc bklog
 891.58
	   0.00
read:          
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		 24.792
	2911.446
	117.434
       0 (0.0%)
	  4.410
	  4.997
write:         
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		  1.486
	242.703
	163.366
       0 (0.0%)
	 10.667
	880.263

10.1.0.21:/var/opt/gitlab/git-data mounted on /var/opt/gitlab/git-data:

   op/s		rpc bklog
 659.80
	   0.00
read:          
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		110.200
	26533.862
	240.779
       0 (0.0%)
	  4.739
	  6.820
write:         
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		  0.400
	  1.035
	  2.588
       0 (0.0%)
	  5.500
	  6.000

10.1.0.21:/var/opt/gitlab/git-data mounted on /var/opt/gitlab/git-data:

   op/s		rpc bklog
 166.40
	   0.00
read:          
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		  1.200
	  6.027
	  5.023
       0 (0.0%)
	  0.500
	  0.500
write:         
  ops/s		   kB/s		  kB/op		retrans		avg RTT (ms)	avg exe (ms)
		  0.000
	  0.000
	  0.000
       0 (0.0%)
	  0.000
	  0.000

With this we will know how many iops we are sending from each client to each mount (I'm limiting to one mount in this sample) and we will be able to learn how the system is behaving.