Registry tool: registry apply should infer the --parent from values set in configuration

[giteshk]

1. Created a registry configuration using

    --registry.address=apigeeregistry.googleapis.com:443 \
    --registry.insecure=0 \
    --registry.project=<gcp-project-name> \
    --registry.location=global

2. Tried to run registry apply command

/tmp % registry apply -f openapi-sample.yaml
  INFO[0000] Applying openapi-sample.yaml uid=92b5f12e
 FATAL[0000] Task failed: apply file openapi-sample.yaml error=invalid project name "": must match "^projects/([A-Za-z0-9-.]+)/locations/global$" uid=92b5f12e

Expected: registry apply should use the current active configuration and the parent parameter should be optional.

[timburks]

Fixed by #783, but let's hold this open until we merge some tests.

[timburks]

Verified with a test change in #784.