Support API Host and authkey external configuration

When running go unit/integration tests - In addition to obtaining configuration from $OPENWHISK_HOME/whisk.properties or from $HOME/.wskprops, allow the apihost and authkey values to be separately specified such that the previously mentioned configuration files are not needed.

Allow the for the possibility of running tests against an external openwhisk instance that's already up and running outside of the current system.

Related issues: #43, #47, #23

Start with the Go unit and integration test configuration paths that read $OPENWHISK_HOME/whisk.properties and $HOME/.wskprops. Trace how apihost and authkey are loaded, then add a way to provide them separately and verify that tests can target an already-running external OpenWhisk instance without those files.

Written by the indexing model from the issue text.