Add support to use default network by multitenant drivers

All existing multitenant drivers requires net-id and subnet-id from neutron, the goal of this blueprint is in next:

Manila should use default network with multitenant drivers if network data is not provided explicitly via share-network.

Details of implementation is TBD.

Addon:

 All existing multitenant drivers requires net-id and subnet-id from neutron, the goal of this blueprint is in next:

Manila should use default share network if no information about share is while share creating. This default share network could be preconfigured for each tenant. In this case while configuring network for tenant administrator can configure share network where share should be by default. If user want it can configure new share network and use it for creating share.

But also besides of default share network which should be configured for each tenant, manila will have two options in config file: default_subnet_name and default_network_name. This options will be used for creating share network if net-id and subnet-id was not specified in share creating request. This change will be used for creating default share network if it is not preconfigured in tenant, but share network is not selected in share creating request.

In other words manila will have 3 variant where get information about share network:

Share network can be selected while creating each share (it is the only way for now)

If no share network is selected then default share network will be selected (each tenant have there own default share network)

If there are no default share network and no share network is selected then default share network will be created using parameters from manila’s config file.

In this way we provide different possibility for configuration. If some tenant don’t make network configuration it’ll have access to default network and default share network will be create automatically. In other case if network is configured, share network for manila should be configured too.

There is also one detail which can be discussed. Now if we not specified subnet/net ids while creating share network, manila will create share with None values in this fields. While creating default share network manila will create it using subnet/net ids from configuration file. Because I can’t imagine situation when we should create share network with no subnet and net ids, I suggest to use values for subnet and net ids from config file for each (not only default) share network if they are not specified in request.