Server Metadata Update (#93)

This commit updates the Server Metadata so it can be parsed and
translated using JSON tags.

The Server Metadata result has been changed to map[string]string to
match the request.

A unit test was added to verify the above and a piece of metadata has
been added to the server creation acceptance test.
diff --git a/acceptance/openstack/compute/v2/compute.go b/acceptance/openstack/compute/v2/compute.go
index da8ff3b..42a2c10 100644
--- a/acceptance/openstack/compute/v2/compute.go
+++ b/acceptance/openstack/compute/v2/compute.go
@@ -243,6 +243,9 @@
 		Networks: []servers.Network{
 			servers.Network{UUID: networkID},
 		},
+		Metadata: map[string]string{
+			"abc": "def",
+		},
 		Personality: servers.Personality{
 			&servers.File{
 				Path:     "/etc/test",