Exporting Ipmi.Path to be set by config. (#2498)

* Exporting Ipmi.Path to be set by config.

Currently "path" is not exported, giving this error when users try to
override the variable via telegraf.conf as per the sample config:

`field corresponding to `path' is not defined in `*ipmi_sensor.Ipmi'`

Exporting the variable solves the problem.

* Updating changelog.

plugins/inputs/ipmi_sensor/ipmi_test.go

@@ -14,7 +14,7 @@ import (
 func TestGather(t *testing.T) {
 	i := &Ipmi{
 		Servers: []string{"USERID:PASSW0RD@lan(192.168.1.1)"},
-		path:    "ipmitool",
+		Path:    "ipmitool",
 	}
 	// overwriting exec commands with mock commands
 	execCommand = fakeExecCommand
@@ -118,7 +118,7 @@ func TestGather(t *testing.T) {
 	}
 
 	i = &Ipmi{
-		path: "ipmitool",
+		Path: "ipmitool",
 	}
 
 	err = i.Gather(&acc)