GCP - Set openshift_gcp_network_name to default if not provided

[tylerauerbeck]

Follow on to #317

Currently in order to use GCP load balancers you have to set openshift_gcp_network_name: default in your inventory. In general, your cluster network will use the default GCP network -- so unless you have a more advanced use case you're not likely to change this.

The problem that I'm trying to solve is if you're in the majority of users where you're just using the default network, you may decide to exclude this or miss having this as part of your inventory. The problem is that if this isn't included/set, you'll be unable to use the gcp load balancer functionality as it won't be configured correctly. What I suggest doing is that if openshift_gcp_network_name isn't set, then we should just have this set to default. This would ensure that things get configured appropriately and wouldn't require folks to know that this is required in order to provide this functionality (as it's not exactly straight forward that this is required for that feature).