Enable flavors metadata update

Nova currently expose an api to let the users update
the flavors metadata so horizon should expose
this functionality. At the same time Glance is exposing
a metadata dictionary in Juno, where users can register
the key/value pairs that describe their cloud deployment.
This patch uses the glance metadata api to expose the available
metadata and the nova api to update the aggregates metadata.

Implements: blueprint glaance-metadata-flavors

Co-Authored-By: Santiago Baldassin <santiago.b.baldassin@intel.com>
Co-Authored-By: Pawel Skowron <pawel.skowron@intel.com>
Co-Authored-By: Pawel Koniszewski <pawel.koniszewski@intel.com>
Co-Authored-By: Michal Dulko <michal.dulko@intel.com>

DocImpact:
    New UI. FLAVOR_EXTRA_KEYS setting deprecated.

Change-Id: Ica9d3d06314a4d5afa77f644bd1f8900a1c328f7