cert-manager: Install cert-manager

*cert-manager* manages certificates.  More specifically, it is an ACME
client, which generates certificate-signing requests, submits them to a
certificate authority, and stores the signed certificate in Kubernetes
secrets.  The certificates it manages are defined by Kubernetes
Custom Resources, either defined manually or automatically for Ingress
resources with particular annotations.

The *cert-manager* deployment consists primarily of two services:
*cert-manager* itself, which monitors Kubernetes resources and manages
certificate requests, and the *cert-manager-webhook*, which validates
Kubernetes resources for *cert-manager*.  There is also a third
component, *cainjector*, we do not need it.

The primary configuration for *cert-manager* is done through Issuer and
ClusterIssuer resources.  These define how certificates are issued: the
certificate authority to use and how to handle ACME challenges.  For our
purposes, we will be using ZeroSSL to issue certificates, verified via
the DNS.01 challenge through BIND running on the gateway firewall.

cert-manager/cluster-issuer.yaml

@@ -0,0 +1,26 @@
+apiVersion: cert-manager.io/v1
+kind: ClusterIssuer
+metadata:
+  name: zerossl
+spec:
+  acme:
+    server: https://acme.zerossl.com/v2/DV90
+    email: zerossl@dustinhatch.me
+    externalAccountBinding:
+      keyID: f06nxtk433oeVdcaDLp9Iw
+      keySecretRef:
+        name: zerossl-eab
+        key: secret
+    privateKeySecretRef:
+      name: zerossl-prod
+
+    solvers:
+    - dns01:
+        cnameStrategy: Follow
+        rfc2136:
+          nameserver: 172.30.0.1
+          tsigKeyName: cert-manager
+          tsigAlgorithm: HMACSHA512
+          tsigSecretSecretRef:
+            name: cert-manager-tsig
+            key: cert-manager.key