no option to override the fixed_subnet when creating a new cluster

[Impact]

When a cluster template sets fixed_network and fixed_subnet and the user tries to create a new cluster using that template and decides to override the network, the fixed_subnet will inherited from the template, leaving an invalid configuration and later Neutron will refuse to allocate a port (since the subnet doesn't belong to the network). For more details see https://bugs.launchpad.net/ubuntu/+source/magnum/+bug/2038109

[Test Case]

1. Create a new cluster template with a fixed_network test-net and fixed_subnet test-subnet
2. Create a new cluster, uncheck the option "Create new network" and pick a network different from test-net in the dropdown list.

Expected result:

The cluster gets created

Actual result:

The cluster creation fails, because the network configuration is invalid.