Fix openstack blockstorage attachments types

This commit changes the blockstorage attachments type from
a slice of strings to a slice of string maps to better suit the
actual returned results.
diff --git a/openstack/blockstorage/v1/volumes/results.go b/openstack/blockstorage/v1/volumes/results.go
index aff60bc..2fd4ef1 100644
--- a/openstack/blockstorage/v1/volumes/results.go
+++ b/openstack/blockstorage/v1/volumes/results.go
@@ -16,7 +16,7 @@
 	Name string `mapstructure:"display_name"`
 
 	// Instances onto which the volume is attached.
-	Attachments []string `mapstructure:"attachments"`
+	Attachments []map[string]interface{} `mapstructure:"attachments"`
 
 	// This parameter is no longer used.
 	AvailabilityZone string `mapstructure:"availability_zone"`