Allow override of discovery URL via ConfigMap

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

- Seems like the next step is to allow override of the CA bundle; I didn't
  do that here for simplicity of the commit, but seems like it is the right
  thing to do in the future.

pkg/config/testdata/happy.yaml

@@ -1,4 +1,6 @@
 ---
+discovery:
+  url: https://some.discovery/url
 webhook:
   url: https://tuna.com/fish?marlin
   caBundle: LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0tLi4u

pkg/config/testdata/no-discovery.yaml

@@ -0,0 +1,5 @@
+---
+webhook:
+  url: https://tuna.com/fish?marlin
+  caBundle: LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0tLi4u
+