action context missing api host in some runtimes

[Rodric Rabbah <ro...@gmail.com>]

I recently observed that some runtimes may not be properly exporting the
__OW_API_HOST context property. Unlike other environment variables, this
one is set by on the container startup (e.g., docker run -e). The runtime
unit test did not catch this because it sends the property in the run
payload and so it gets exported as any other context property. Arguably we
should change the invoker protocol to send this value as it does the others
(it is done at container startup time since it's fixed for all containers
so why serialize it).

I have adjusted the test [1] and opened a patch against one runtime that
needs to be adjusted as a result [2].
[1] https://github.com/apache/openwhisk/pull/4765
[2] https://github.com/apache/openwhisk-runtime-go/pull/115

-r