Add support for master_lb_enabled field for containerinfra cluster resource

Adds a field to enable/disable load balancer for magnum cluster master node.

Link to code: [magnum/api/controllers/v1/cluster.py#L205](https://github.com/openstack/magnum/blob/1cdc0628a28da32ef52324244a4c97b0794c2542/magnum/api/controllers/v1/cluster.py#L205)

Link to commit where the feature was added: https://github.com/openstack/magnum/commit/946c1d67c73a1b325dadd27018c38555acfee52f