Add setting to avoid subscription creation (#15)

Add a configuration property that will skip attempting to create the
pubsub subscription.

For #13

Note the default subscriber role still will not work, minimal
permissions can be granted as described here
googleapis/google-cloud-node#1502 (comment).

CONTRIBUTORS

@@ -0,0 +1,13 @@
+The following is a list of people who have contributed ideas, code, bug
+reports, or in general have helped pubsubbeat along its way.
+
+Contributors:
+
+* Chris Koehnke ([email protected])
+* Joseph Lewis III ([email protected])
+* Vincent Roseberry ([email protected])
+
+Note: If you've sent us patches, bug reports, or otherwise contributed to
+pubsubbeat, and you aren't on the list above and want to be, please let us know
+and we'll make sure you're here. Contributions from folks like you are what make
+open source awesome.

beater/pubsubbeat.go

@@ -159,6 +159,11 @@ func createPubsubClient(config *config.Config) (*pubsub.Client, error) {
 }
 
 func getOrCreateSubscription(client *pubsub.Client, config *config.Config) (*pubsub.Subscription, error) {
+	if !config.Subscription.Create {
+		subscription := client.Subscription(config.Subscription.Name)
+		return subscription, nil
+	}
+
 	topic := client.Topic(config.Topic)
 	ctx := context.Background()
 

config/config.go

@@ -32,14 +32,21 @@ type Config struct {
 		Name                string        `config:"name" validate:"required"`
 		RetainAckedMessages bool          `config:"retain_acked_messages"`
 		RetentionDuration   time.Duration `config:"retention_duration"`
+		Create              bool          `config:"create"`
 	}
 	Json struct {
 		Enabled     bool `config:"enabled"`
 		AddErrorKey bool `config:"add_error_key"`
 	}
 }
 
-var DefaultConfig = Config{}
+func GetDefaultConfig() Config {
+	config := Config{}
+	config.Subscription.Create = true
+	return config
+}
+
+var DefaultConfig = GetDefaultConfig()
 
 func GetAndValidateConfig(cfg *common.Config) (*Config, error) {
 	c := DefaultConfig

pubsubbeat.yml

@@ -21,10 +21,12 @@ pubsubbeat:
   # The Pub/Sub topic name. You must first create this topic.
   topic: my-topic
 
-  # The Pub/Sub subscription name. If the subscription does not exist,
-  # Pubsubbeat will attempt to create the subscription.
+  # The Pub/Sub subscription name.
   subscription.name: my-subscription
 
+  # Attempt to create the subscription.
+  subscription.create: true # Defaults to true
+
   # The settings below are used only if the Beat creates the subscription.
 
   # Whether to retain acknowledged messages. If true, acknowledged messages