Add a switch environment var to disable GUI components.
Add a switch environment var to disable GUI components.

- Also disables requiring VESPA_VAGRANT_VM_BOX_URL to allow
  using boxes from Vagrant cloud.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa