fn: introducing docker-syslog driver as default logger (#1189)

* fn: introducing docker-syslog driver as default logger

With this change, fn-agent prefers RFC2454 docker-syslog driver
for logging stdout/stderr from containers. The advantage
of this is to offload it to docker itself instead of
streaming stderr along with stdout, which gets multiplexed
through single connection via docker-API.

The change will need support from FDKs in order to log
correct call-id and supress '\n' that splits syslog lines.

api/agent/drivers/driver.go

@@ -86,6 +86,21 @@ type RunResult interface {
 	Status() string
 }
 
+// Logger Tags for container
+type LoggerTag struct {
+	Name  string
+	Value string
+}
+
+// Logger Configuration for container
+type LoggerConfig struct {
+	// Log Sink URL
+	URL string
+
+	// Log Tag Pairs
+	Tags []LoggerTag
+}
+
 // The ContainerTask interface guides container execution across a wide variety of
 // container oriented runtimes.
 type ContainerTask interface {
@@ -136,6 +151,9 @@ type ContainerTask interface {
 	// leaves it unset.
 	WorkDir() string
 
+	// Logger Config to use in driver
+	LoggerConfig() LoggerConfig
+
 	// Close is used to perform cleanup after task execution.
 	// Close should be safe to call multiple times.
 	Close()