Get OpenStack variables from env as fallback (#3293)

This change enables the OpenStack service discovery to read the
authentication parameters from the OS_* environment variables when the
identity endpoint URL is not defined in the Prometheus configuration
file.

discovery/openstack/openstack.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/go-kit/kit/log"
 	"github.com/gophercloud/gophercloud"
+	"github.com/gophercloud/gophercloud/openstack"
 	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/net/context"
 
@@ -52,7 +53,15 @@ type Discovery interface {
 
 // NewDiscovery returns a new OpenStackDiscovery which periodically refreshes its targets.
 func NewDiscovery(conf *config.OpenstackSDConfig, l log.Logger) (Discovery, error) {
-	opts := gophercloud.AuthOptions{
+	var opts gophercloud.AuthOptions
+	if conf.IdentityEndpoint == "" {
+		var err error
+		opts, err = openstack.AuthOptionsFromEnv()
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		opts = gophercloud.AuthOptions{
 			IdentityEndpoint: conf.IdentityEndpoint,
 			Username:         conf.Username,
 			UserID:           conf.UserID,
@@ -62,6 +71,7 @@ func NewDiscovery(conf *config.OpenstackSDConfig, l log.Logger) (Discovery, erro
 			DomainName:       conf.DomainName,
 			DomainID:         conf.DomainID,
 		}
+	}
 	switch conf.Role {
 	case config.OpenStackRoleHypervisor:
 		hypervisor := NewHypervisorDiscovery(&opts,