Initial journald log provider attempt

**What**
- journald log provider using exec to journalctl
```
journalctl -t <namespace>:<name>  --output=json --since=<timestamp> <--follow> --output-fields=SYSLOG_IDENTIFIER,MESSAGE,_PID,_SOURCE_REALTIME_TIMESTAMP
```
- This can be tested manually using `faas-cli logs` as normal, e.g.
  `faas-cli logs nodeinfo` should tail the last 5 mins of logs.
- Very basic tests ensuring that the `journalctl` comamand is correctly
  construction and that the json log entrys are parsed correctly.
- Add simple e2e test to grep the function logs

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

pkg/logs/requestor_test.go

@@ -0,0 +1,73 @@
+package logs
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/openfaas/faas-provider/logs"
+)
+
+func Test_parseEntry(t *testing.T) {
+	rawEntry := `{ "__CURSOR" : "s=71c4550142d14ace8e2959e3540cc15c;i=133c;b=44864010f0d94baba7b6bf8019f82a56;m=2945cd3;t=5a00d4eb59180;x=8ed47f7f9b3d798", "__REALTIME_TIMESTAMP" : "1583353899094400", "__MONOTONIC_TIMESTAMP" : "43277523", "_BOOT_ID" : "44864010f0d94baba7b6bf8019f82a56", "SYSLOG_IDENTIFIER" : "openfaas-fn:nodeinfo", "_PID" : "2254", "MESSAGE" : "2020/03/04 20:31:39 POST / - 200 OK - ContentLength: 83", "_SOURCE_REALTIME_TIMESTAMP" : "1583353899094372" }`
+	expectedEntry := logs.Message{
+		Name:      "nodeinfo",
+		Namespace: "openfaas-fn",
+		Text:      "2020/03/04 20:31:39 POST / - 200 OK - ContentLength: 83",
+		Timestamp: time.Unix(0, 1583353899094400*1000).UTC(),
+	}
+
+	value := map[string]string{}
+	json.Unmarshal([]byte(rawEntry), &value)
+
+	entry, err := parseEntry(value)
+	if err != nil {
+		t.Fatalf("unexpected error %s", err)
+	}
+
+	if entry.Name != expectedEntry.Name {
+		t.Fatalf("expected Name %s, got %s", expectedEntry.Name, entry.Name)
+	}
+
+	if entry.Namespace != expectedEntry.Namespace {
+		t.Fatalf("expected Namespace %s, got %s", expectedEntry.Namespace, entry.Namespace)
+	}
+
+	if entry.Timestamp != expectedEntry.Timestamp {
+		t.Fatalf("expected Timestamp %s, got %s", expectedEntry.Timestamp, entry.Timestamp)
+	}
+
+	if entry.Text != expectedEntry.Text {
+		t.Fatalf("expected Text %s, got %s", expectedEntry.Text, entry.Text)
+	}
+}
+
+func Test_buildCmd(t *testing.T) {
+	ctx := context.TODO()
+	now := time.Now()
+	req := logs.Request{
+		Name:      "loggyfunc",
+		Namespace: "spacetwo",
+		Follow:    true,
+		Since:     &now,
+		Tail:      5,
+	}
+
+	expectedArgs := fmt.Sprintf(
+		"--utc --no-pager --output=json --output-fields=SYSLOG_IDENTIFIER,MESSAGE,_PID,_SOURCE_REALTIME_TIMESTAMP --identifier=spacetwo:loggyfunc --since=%s --follow --lines=5",
+		now.UTC().Format("2006-01-02 15:04:05"),
+	)
+
+	cmd := buildCmd(ctx, req).String()
+
+	if !strings.Contains(cmd, "journalctl") {
+		t.Fatalf("expected journalctl cmd, got cmd %s", cmd)
+	}
+
+	if !strings.HasSuffix(cmd, expectedArgs) {
+		t.Fatalf("expected arg %s,\ngot cmd %s", expectedArgs, cmd)
+	}
+}