add sslrootcert to database options conditionally (fixes #1215)

roles/installer/templates/settings/credentials.py.j2

@@ -8,7 +8,9 @@ DATABASES = {
         'HOST': '{{ awx_postgres_host }}',
         'PORT': "{{ awx_postgres_port }}",
         'OPTIONS': { 'sslmode': '{{ awx_postgres_sslmode }}',
+                     {%- if awx_postgres_sslmode == 'verify-ca' or awx_postgres_sslmode == 'verify-full' %}
                      'sslrootcert': '{{ ca_trust_bundle }}',
+                     {%- endif %}
         },
     }
 }