nova-metadata-aggregates

Nova client lets you update the metadata of a host aggregate via the `nova aggregate-set-metadata`. We should let the user do this via the UI as well.

Note: There are other blueprints open around the ability to update metadata (e.g. https://blueprints.launchpad.net/horizon/+spec/horizon-create-image-metadata ), it may be worthwhile to try and keep the user experience consistent.

$ nova help aggregate-set-metadata
usage: nova aggregate-set-metadata <aggregate> <key=value> [<key=value> ...]

Update the metadata associated with the aggregate.

Positional arguments:
  <aggregate> Name or ID of aggregate to update.
  <key=value> Metadata to add/update to aggregate