Use private resource controller endpoint in DA by default

[ocofaigh]

ibmcloud_resource_controller_api_endpoint should default to "https://private.resource-controller.cloud.ibm.com" but its currently failing to resolve in schematics:

dial tcp: lookup private.resource-controller.cloud.ibm.com on 172.21.0.10:53: no such host

Have reached out to schematics team - keeping this open to track

[ocofaigh]

As per docs, need to use on of these:

Dallas: https://private.us-south.resource-controller.cloud.ibm.com/
Washington DC: https://private.us-east.resource-controller.cloud.ibm.com/

[ocofaigh]

#248