Cinder supports filter volumes with metadata, but in some case,
users don't know what metadata the volumes contain or what metadata
is valid to filter volumes.
This patch updated volumes/summary API to return all valid distinct
metadata to users. Then users could use these metadatas to filter
volumes easily.
This function is useful for dashboard, such as Horizon, as well.
APIImpact
Implements: blueprint metadata-for-volume-summary
Change-Id: I33c77d9db88f70d8d3b8ea86c86c01220dcc537c
bf40945dcc