Make net and subnet name customization optional

https://github.com/openstack-charmers/zaza-openstack-tests/commit/ad16b5a7fc3a0971b168b133fbf0b5cbd55115fd

ad16b5a7fc introduced an option to
customize network and subnet names, however, not all modules were
updated to pass it into the setup_sdn method. Specificatlly, the
dragent now breaks with a KeyError during the configure step.

(Pdb) network_config["project_net_name"]
*** KeyError: 'project_net_name'

https://github.com/openstack-charmers/zaza-openstack-tests/blob/fdaf43f52c31656e445561477a7b7c468a73b89a/zaza/openstack/charm_tests/dragent/configure.py#L35-L46

zaza/openstack/configure/network.py

@@ -167,7 +167,7 @@ def setup_sdn(network_config, keystone_session=None):
         project_id,
         shared=False,
         network_type=network_config["network_type"],
-        net_name=network_config["project_net_name"])
+        net_name=network_config.get("project_net_name", "private"))
     project_subnet = openstack_utils.create_project_subnet(
         neutron_client,
         project_id,
@@ -175,7 +175,8 @@ def setup_sdn(network_config, keystone_session=None):
         network_config.get("private_net_cidr"),
         subnetpool=subnetpool,
         ip_version=ip_version,
-        subnet_name=network_config["project_subnet_name"])
+        subnet_name=network_config.get("project_subnet_name",
+                                       "private_subnet"))
     openstack_utils.update_subnet_dns(
         neutron_client,
         project_subnet,