cluster/admitomatic: allow use-regex n-i-c annotation

This annotation is used to permit routes defined by regexes instead of
simple prefix matching. This is used by our synapse deployment for
routing incomming HTTP requests to diffferent Synapse components.

I've stumbled upon this while deploying a new Matrix/Synapse instance.
This hasn't been yet a problem because the existing ingresses for Matrix
deployments predate admitomatic.

Change-Id: I821e58b214450ccf0de22d2585c3b0d11fbe71c0
diff --git a/cluster/admitomatic/ingress.go b/cluster/admitomatic/ingress.go
index 6b8a365..22e9dab 100644
--- a/cluster/admitomatic/ingress.go
+++ b/cluster/admitomatic/ingress.go
@@ -210,6 +210,7 @@
 		"proxy-body-size":  true,
 		"ssl-redirect":     true,
 		"backend-protocol": true,
+		"use-regex":        true,
 		// Used by cert-manager
 		"whitelist-source-range": true,
 	}