move block storage extensions from 'v2' to 'blockstorage' (#112)

diff --git a/acceptance/openstack/blockstorage/extensions/pkg.go b/acceptance/openstack/blockstorage/extensions/pkg.go
new file mode 100644
index 0000000..f18039d
--- /dev/null
+++ b/acceptance/openstack/blockstorage/extensions/pkg.go
@@ -0,0 +1,3 @@
+// The extensions package contains acceptance tests for the Openstack Cinder extensions service.
+
+package extensions