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>

vendor/github.com/openfaas/faas-provider/logs/logs.go

@@ -0,0 +1,62 @@
+// Package logs provides the standard interface and handler for OpenFaaS providers to expose function logs.
+//
+// The package defines the Requester interface that OpenFaaS providers should implement and then expose using
+// the predefined NewLogHandlerFunc. See the example folder for a minimal log provider implementation.
+//
+// The Requester is where the actual specific logic for connecting to and querying the log system should be implemented.
+//
+package logs
+
+import (
+	"fmt"
+	"time"
+)
+
+// Request is the query to return the function logs.
+type Request struct {
+	// Name is the function name and is required
+	Name string `json:"name"`
+	// Namespace is the namespace the function is deployed to, how a namespace is defined
+	// is faas-provider specific
+	Namespace string `json:"namespace"`
+	// Instance is the optional container name, that allows you to request logs from a specific function instance
+	Instance string `json:"instance"`
+	// Since is the optional datetime value to start the logs from
+	Since *time.Time `json:"since"`
+	// Tail sets the maximum number of log messages to return, <=0 means unlimited
+	Tail int `json:"tail"`
+	// Follow is allows the user to request a stream of logs until the timeout
+	Follow bool `json:"follow"`
+}
+
+// String implements that Stringer interface and prints the log Request in a consistent way that
+// allows you to safely compare if two requests have the same value.
+func (r Request) String() string {
+	return fmt.Sprintf(
+		"name:%s namespace: %s instance:%s since:%v tail:%d follow:%v",
+		r.Name, r.Namespace, r.Instance, r.Since, r.Tail, r.Follow,
+	)
+}
+
+// Message is a specific log message from a function container log stream
+type Message struct {
+	// Name is the function name
+	Name string `json:"name"`
+	// Namespace is the namespace the function is deployed to, how a namespace is defined
+	// is faas-provider specific
+	Namespace string `json:"namespace"`
+	// instance is the name/id of the specific function instance
+	Instance string `json:"instance"`
+	// Timestamp is the timestamp of when the log message was recorded
+	Timestamp time.Time `json:"timestamp"`
+	// Text is the raw log message content
+	Text string `json:"text"`
+}
+
+// String implements the Stringer interface and allows for nice and simple string formatting of a log Message.
+func (m Message) String() string {
+	return fmt.Sprintf(
+		"%s %s (%s %s) %s",
+		m.Timestamp.String(), m.Name, m.Namespace, m.Instance, m.Text,
+	)
+}