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

[shazron]

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 apache/openwhisk-cli#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 ACNA-296 - Remove namespace from .wskprops #80)

[adobe-bot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-596