Add tags for OpenStack resources

Currently, the EC2 API of OpenStack only has tags support (metadata) for instances. And there has already a blueprint about to add support for volumes and volume snapshots using “metadata”.
In OpenStack, generally, the choice was made to use maps of key/value pairs instead of lists of strings(tags) to annotate objects exposed in the REST APIs. OpenStack REST APIs inconsistently call these maps of key/value pairs:
  * "properties" (Glance, Cinder Image, Volume,respectively)
  * "extra_specs" (Nova InstanceType)
  * "metadata" (Nova Instance, Aggregate and InstanceGroup, Neutron)
  * "metadetails" (Nova Aggregate and InstanceGroup)
  * "system_metadata" (Nova Instance -- differs from "normal" metadata in that the key/value pairs are 'owned' by Nova, not a user...)
Tags are a cleaner way of annotating objects when the annotation is coming from a normal user. Tags represent by far the most common way for REST APIs to enable user-facing annotation of objects in a way that is easy to search on. We should support for tags added to any searchable/queryable object in all of the OpenStack APIs.
And the key/value pairs can only be changed by users with elevated privileged system-controlled, the normal users should use tags.