Add configuration directive to allow/disallow duplicate named bay/baymodel

Add two configuration directives 'allow_duplicate_baymodel_name' and 'allow_duplicate_baymodel_name', default values are FALSE. If TRUE,
duplicate named Bay/BayModel resources would be allowed, as they are
today. If FALSE, the name of each bay/baymodel need to be unique.

This way, by default Magnum requires a unique name, and cloud operators can decide
if they want to enforce name uniqueness or not.

In the case of clouds that want to allow sharing access to a BayModel between multiple tenants (example: a global BayModel named “kubernetes”) with 'allow_duplicate_baymodel_name' set to FALSE, a user will still be allowed to create a BayModel with the name “kubernetes” and it will override the global one. If a user-supplied BayModel is presented with the same name as a global one, the one owned by the tenant would be selected automatically.

This feature was proposed in blueprint auto-generate-name [1] first, but that bp proposed
two different features so I moved one of the features to this blueprint.

[1]https://blueprints.launchpad.net/magnum/+spec/auto-generate-name