config: parse environment variables in the config file

closes #663

internal/config/config_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"os"
 	"testing"
 	"time"
 
@@ -10,9 +11,52 @@ import (
 	"github.com/influxdata/telegraf/plugins/inputs/memcached"
 	"github.com/influxdata/telegraf/plugins/inputs/procstat"
 	"github.com/influxdata/telegraf/plugins/parsers"
+
 	"github.com/stretchr/testify/assert"
 )
 
+func TestConfig_LoadSingleInputWithEnvVars(t *testing.T) {
+	c := NewConfig()
+	err := os.Setenv("MY_TEST_SERVER", "192.168.1.1")
+	assert.NoError(t, err)
+	err = os.Setenv("TEST_INTERVAL", "10s")
+	assert.NoError(t, err)
+	c.LoadConfig("./testdata/single_plugin_env_vars.toml")
+
+	memcached := inputs.Inputs["memcached"]().(*memcached.Memcached)
+	memcached.Servers = []string{"192.168.1.1"}
+
+	mConfig := &internal_models.InputConfig{
+		Name: "memcached",
+		Filter: internal_models.Filter{
+			NameDrop:  []string{"metricname2"},
+			NamePass:  []string{"metricname1"},
+			FieldDrop: []string{"other", "stuff"},
+			FieldPass: []string{"some", "strings"},
+			TagDrop: []internal_models.TagFilter{
+				internal_models.TagFilter{
+					Name:   "badtag",
+					Filter: []string{"othertag"},
+				},
+			},
+			TagPass: []internal_models.TagFilter{
+				internal_models.TagFilter{
+					Name:   "goodtag",
+					Filter: []string{"mytag"},
+				},
+			},
+			IsActive: true,
+		},
+		Interval: 10 * time.Second,
+	}
+	mConfig.Tags = make(map[string]string)
+
+	assert.Equal(t, memcached, c.Inputs[0].Input,
+		"Testdata did not produce a correct memcached struct.")
+	assert.Equal(t, mConfig, c.Inputs[0].Config,
+		"Testdata did not produce correct memcached metadata.")
+}
+
 func TestConfig_LoadSingleInput(t *testing.T) {
 	c := NewConfig()
 	c.LoadConfig("./testdata/single_plugin.toml")