Enable host aggregate metadata update

Nova currently expose an api to let the users update
the host aggregates 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

Co-Authored-By: Santiago Baldassin <santiago.b.baldassin@intel.com>
Co-Authored-By: Pawel Skowron <pawel.skowron@intel.com>
Co-Authored-By: Bartosz Fic <bartosz.fic@intel.com>
Co-Authored-By: Michal Dulko <michal.dulko@intel.com>
Co-Authored-By: Pawel Koniszewski <pawel.koniszewski@intel.com>
DocImpact
Change-Id: I4abf88426f548c47a21920169f710d3310e3ede6
Implements: blueprint host-aggregate-update-metadata
Merge "Enable host aggregate metadata update"