Use default namespace `_` instead of the actual namespace name in API calls

This is for the urls being called to Runtime.

Setting the namespace is a remnant where an auth key can map to multiple namespaces, now there is a 1:1 mapping. See https://github.com/apache/openwhisk-cli/pull/434

Enforce these limitations:
1. Replace with `_` only in API calls
2. Do not replace when generating action web-urls
3. Keep `NAMESPACE` in .wskprops for informational purposes (see #80)

Start by tracing how the CLI constructs URLs for Runtime API calls and separately how it generates action web-URLs. Check how NAMESPACE is read from .wskprops, then verify that API requests use the default `_`, web-URLs retain the actual namespace, and NAMESPACE remains available for informational purposes.

Written by the indexing model from the issue text.