Add a custom log formatter

It just keeps the log entry message and appends a newline to it.
This makes them more user-friendly in the terminal.

Before:

    INFO[0000] Log message.

After:

    Log message.

https://jira.mesosphere.com/browse/DCOS_OSS-3746

pkg/log/formatter_test.go

@@ -0,0 +1,48 @@
+package log
+
+import (
+	"bytes"
+	"io/ioutil"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/sirupsen/logrus"
+)
+
+func TestFormat(t *testing.T) {
+	formatter := &Formatter{}
+	logger := &logrus.Logger{
+		Out:       ioutil.Discard,
+		Formatter: formatter,
+	}
+
+	fixtures := []struct {
+		entry  *logrus.Entry
+		expOut string
+	}{
+		{
+			// simple entry
+			&logrus.Entry{
+				Logger:  logger,
+				Message: "DEBUGME",
+			},
+			"DEBUGME\n",
+		},
+		{
+			// entry with buffer
+			&logrus.Entry{
+				Logger:  logger,
+				Message: "DEBUGMETOO",
+				Buffer:  bytes.NewBuffer(nil),
+			},
+			"DEBUGMETOO\n",
+		},
+	}
+
+	for _, fixture := range fixtures {
+		out, err := formatter.Format(fixture.entry)
+		require.NoError(t, err)
+		require.Equal(t, fixture.expOut, string(out))
+	}
+}