Change hddtemp to always put temperature in temperature field (#1905)

Added unit tests for the changes

Fixes #1904

plugins/inputs/hddtemp/README.md

@@ -8,7 +8,7 @@ Hddtemp should be installed and its daemon running
 
 ## Configuration
 
-```
+```toml
 [[inputs.hddtemp]]
 ## By default, telegraf gathers temps data from all disks detected by the
 ## hddtemp.
@@ -20,3 +20,24 @@ Hddtemp should be installed and its daemon running
 # address = "127.0.0.1:7634"
 # devices = ["sda", "*"]
 ```
+
+## Measurements
+
+- hddtemp
+  - temperature
+
+Tags:
+- device
+- model
+- unit
+- status
+
+
+
+## Example output
+
+```
+> hddtemp,unit=C,status=,host=server1,device=sdb,model=WDC\ WD740GD-00FLA1 temperature=43i 1481655647000000000
+> hddtemp,device=sdc,model=SAMSUNG\ HD103UI,unit=C,status=,host=server1 temperature=38i 148165564700000000
+> hddtemp,device=sdd,model=SAMSUNG\ HD103UI,unit=C,status=,host=server1 temperature=36i 1481655647000000000
+```