Add namespace field to activation log

[蒋鹏程]

Dear all:
​
 While I was using DockerToActivationFileLogStore ​to store acitavation logs to 
a separate file and use logstash to process this file, I found it's not 
convenient to send logs to different indices in ElasticSearch using namespace
as a variable, such as send to "openwhisk-%{namespace}", so I added a filed 
"namespace" in this pr: https://github.com/apache/openwhisk/pull/4609, it's a 
small change, Please help and comment on the PR.
​
Thanks
Jiang PengCheng