config: add top-level YAML namespace 'clair'

This enables Clair to use YAML configuration files that contain more
than just a Clair configuration.

Fixes #95.

config.example.yaml

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 # The values specified here are the default values that Clair uses if no configuration file is specified or if the keys are not defined.
----
+clair:
   database:
     # PostgreSQL Connection string
     # http://www.postgresql.org/docs/9.4/static/libpq-connect.html
@@ -43,6 +43,7 @@ api:
     # If you want to easily generate client certificates and CAs, try the following projects:
     # https://github.com/coreos/etcd-ca
     # https://github.com/cloudflare/cfssl
+    servername:
     cafile:
     keyfile:
     certfile:
@@ -65,8 +66,8 @@ notifier:
 
       # Optional PKI configuration
       # If you want to easily generate client certificates and CAs, try the following projects:
-    # https://github.com/coreos/etcd-ca
       # https://github.com/cloudflare/cfssl
+      # https://github.com/coreos/etcd-ca
       servername:
       cafile:
       keyfile:

config/config.go

@@ -23,6 +23,12 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+// File represents a YAML configuration file that namespaces all Clair
+// configuration under the top-level "clair" key.
+type File struct {
+	Clair Config `yaml:"clair"`
+}
+
 // Config is the global configuration for an instance of Clair.
 type Config struct {
 	Database *DatabaseConfig
@@ -97,11 +103,14 @@ func Load(path string) (config *Config, err error) {
 		return
 	}
 
-	err = yaml.Unmarshal(d, config)
+	var cfgFile File
+	err = yaml.Unmarshal(d, &cfgFile)
 	if err != nil {
 		return
 	}
+	config = &cfgFile.Clair
 
+	// Generate a pagination key if none is provided.
 	if config.API.PaginationKey == "" {
 		var key fernet.Key
 		if err = key.Generate(); err != nil {