Not specifying network-config.mtu with type=canal results in invalid CNI configuration

[colhom]

Is this a BUG REPORT or FEATURE REQUEST?

BUG REPORT

Versions

Tectonic version (release or commit hash):

master (c16b1ce6af65b2eb94b43479996f32be0e472a5b)

Platform (aws|azure|openstack|metal|vmware):

aws

What happened?

In the tectonic config yaml, the `network-config.mtu` field is marked as optional. If the default network `type=canal` is used **and** the `mtu` field is omitted, the cluster-config-v1 configmap does not specify an mtu for calico. 

Consequently calico CNI configuration JSON is invalid. 

{
   "mtu" : ,
}

The end result is that the calico pods fail to provide pod networking, as they cannot parse the CNI JSON file.

What you expected to happen?

If the MTU field is omitted from the tectonic CLI config, a sane default should be used and a valid CNI configuration should still be generated. This number should most likely be 1440. 

How to reproduce it (as minimally and precisely as possible)?

Install a Tectonic cluster with tectonic CLI tool, but do not specify any network options. This will result in type=canal networking. If you look at configmap/cluster-config-v1, you will notice that the MTU for calico is not specified. If you look at the kube-calico configmap, you will also see that CNI configuration JSON is malformed as show above.