Deprecate nova-manage flavor subcommand

The nova client already has the following flavor commands:

flavor-access-add    Add flavor access for the given tenant.
flavor-access-list   Print access information about the given flavor.
flavor-access-remove Remove flavor access for the given tenant.
flavor-create        Create a new flavor
flavor-delete        Delete a specific flavor
flavor-key           Set or unset extra_spec for a flavor.
flavor-list          Print a list of available 'flavors' (sizes of
flavor-show          Show details about the given flavor.

This covers what nova-manage flavor was providing so let's deprecate the
legacy nova-manage subcommand and mark it for removal in the K release.

There is not a bug or blueprint for this. The flavor subcommand is
completely duplicated in nova client. It is accessing the database
directly and we want to move to using flavor objects which would go
through conductor. So this could be considered part of the objects
conversion for flavors.

For some other commands like service, db and host it makes sense to
leave those so you can access the database before the api service is
running, but that's not the case for flavors.

UpgradeImpact: nova-manage flavor command is deprecated, use nova client

Change-Id: I8709bc9af4e490142e44c6dc61fd0aaa3a392b68
Merge "Deprecate nova-manage flavor subcommand"