Ignore boring filesystems from disk plugin

[PierreF]

Modern Linux has a lots of boring filesystem (tmpfs on /dev, devpts on /dev/pts, lots of cgroup on /sys/fs/cgroup/*, ...).

This pull request:

• Ignore filesystem with 0 bytes (this cover cgroup, devpts and other).
• Add IgnoreFS configuration option to ignore additional FS by their type. Add tmpfs and
  devtmpfs as default ignored type.

With telegraf 0.10.3, on my laptop I've 34 disk monitored !

$ telegraf -version
Telegraf - Version 0.10.3
$ telegraf -sample-config -input-filter disk -output-filter influxdb > test.conf
$ telegraf -config test.conf -test|wc -l
35
$ telegraf -config test.conf -test
* Plugin: disk, Collection 1
> disk,fstype=sysfs,path=/sys free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1456162583021865183
> disk,fstype=proc,path=/proc free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1456162583021903499
> disk,fstype=devtmpfs,path=/dev free=4117389312i,inodes_free=1004598i,inodes_total=1005222i,inodes_used=624i,total=4117389312i,used=0i,used_percent=0 1456162583021929217
[...]
> disk,fstype=ext4,path=/home free=15607943168i,inodes_free=7812422i,inodes_total=9601776i,inodes_used=1789354i,total=155572985856i,used=133537521664i,used_percent=89.5350869799621 1456162583022424585
> disk,fstype=tmpfs,path=/run/cgmanager/fs free=102400i,inodes_free=1009804i,inodes_total=1009817i,inodes_used=13i,total=102400i,used=0i,used_percent=0 1456162583022447295
> disk,fstype=binfmt_misc,path=/proc/sys/fs/binfmt_misc free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1456162583022463800
> disk,fstype=tmpfs,path=/run/user/1000 free=827158528i,inodes_free=1009779i,inodes_total=1009817i,inodes_used=38i,total=827244544i,used=86016i,used_percent=0.0103978926937474 1456162644453701250
> disk,fstype=fuse.gvfsd-fuse,path=/run/user/1000/gvfs free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1456162644453718581

After this patch, the list is reduced to more useful length:

$ ./bin/telegraf -version
Telegraf - Version 0.10.3-14-g86093ae
$ ./bin/telegraf -sample-config -input-filter disk -output-filter influxdb > test.conf
$ ./bin/telegraf -config test.conf -test
* Plugin: disk, Collection 1
> disk,fstype=ext4,path=/ free=11217313792i,inodes_free=1434426i,inodes_total=2239600i,inodes_used=805174i,total=36132761600i,used=23335747584i,used_percent=67.53597700089358 1456162785532394598
> disk,fstype=ext4,path=/boot free=106813440i,inodes_free=62201i,inodes_total=62496i,inodes_used=295i,total=243577856i,used=119778304i,used_percent=52.86084209670058 1456162785532438342
> disk,fstype=vfat,path=/boot/efi free=254140416i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=520093696i,used=265953280i,used_percent=51.135647681451616 1456162785532464010
> disk,fstype=ext4,path=/home free=15605620736i,inodes_free=7812367i,inodes_total=9601776i,inodes_used=1789409i,total=155572985856i,used=133539844096i,used_percent=89.53664413894286 1456162785532482106

[sparrc]

thanks @PierreF, this looks like a very good idea, I'll work on getting this merged. (currently having some very annoying toml parsing problems with our configuration library that is causing the panic, but I'll hopefully get this working soon).