Rationalize how defaults are used when constructing Mixer and tracing cobra args (#3834)
Rationalize how defaults are used when constructing Mixer and tracing cobra args (#3834)

* Rationalize how defaults are used when constructing Mixer and tracing cobra args.

This uses the same pattern as pkg/log and other places to avoid specifying default
values in two places.

* A bit of renaming for consistency.

istio/pkg,istio/pkg,istio/pkg,istio/pkg,istio/pkg