Use empty string for overcloud InternalTLSCAFile param

Not all deployments have the file in the current default location
and rather use trusted certs for public tls. This also creates
issues in downstream jobs that don't inject overcloud ca with
environment/inject-trust-anchor.yaml

This default will ensure that it works in those scenarios.

Change-Id: Ib71c3e2be2b8dc57f3c9107c6ddab47cd6594202
Related-Bug: #1880936

environments/ssl/enable-tls.yaml

@@ -16,7 +16,7 @@ parameter_defaults:
 
   # Specifies the default CA cert to use if TLS is used for services in the internal network.
   # Type: string
-  InternalTLSCAFile: /etc/pki/ca-trust/source/anchors/overcloud-cacert.pem
+  InternalTLSCAFile: ''
 
   # The content of the SSL certificate (without Key) in PEM format.
   # Type: string

sample-env-generator/ssl.yaml

@@ -27,7 +27,7 @@ environments:
         |
             The contents of the private key go here
       HorizonSecureCookies: True
-      InternalTLSCAFile: /etc/pki/ca-trust/source/anchors/overcloud-cacert.pem
+      InternalTLSCAFile: ''
   -
     name: ssl/enable-internal-tls
     title: Enable SSL on OpenStack Internal Endpoints