Add env and env-file flags for action create/update

[giusdp]

Following the PR that adds the Env field to the action struct in client-go apache/openwhisk-client-go#158, this PR adds the --env and --env-file flag to wsk action create/update.

It follows closely the code for the --params and --annotation flags to add support for passing environment variables during action initialization.

I tried to extend some tests in Go but let me know where I should add more.

I've also updated the client-go dep to the latest commit and run go mod tidy which removed many deps.

[kcexn]

Can you explain to me how the environment variables are passed to the action initialization route in the action-proxy?

I thought the OpenWhisk-CLI simply sends requests to the OpenWhisk API? And as far as I can tell, the OpenWhisk API does not directly support passing environment variables to actions on initialization. Unless there is an undocumented annotation for it?