[role:cert-manager] Add self-signed issuer and certificate

For a private PKI, created a custom root certificate using
self-signed clusterIssuer.
To use this for openstack endpoints, need to set variable
`cert_manager_issuer` as following;
cert_manager_issuer:
  ca:
    secretName: root-secret

Sem-Ver: feature

Change-Id: Ie8f46173f7951c141053ad3cf80a5d8926c95724
diff --git a/roles/cert_manager/defaults/main.yml b/roles/cert_manager/defaults/main.yml
index 508c527..40c504f 100644
--- a/roles/cert_manager/defaults/main.yml
+++ b/roles/cert_manager/defaults/main.yml
@@ -20,6 +20,7 @@
 # .. envvar:: cert_manager_issuer [[[
 #
 # Definition for the ``cert-manager`` issuer
+# To use self-signed CA certificate, set cert_manager_issuer.ca.secretName as root-secret.
 cert_manager_issuer:
   acme:
     email: mnaser@vexxhost.com