Upgrade google_container_node_pool() resource to expose 'release channel'

[sukchomb-zz]

We would like to test the release channel of a NodePool resource.

Detailed Description

The release channel is exposed in the response in api Method: projects.locations.clusters.get , and comes out as:

"releaseChannel": {
"channel": "STABLE"
},

Context

Being able to test this property will strengthen the security posture of the platform.

Possible Implementation

[sukchomb-zz]

@skpaterson @slevenick Please have a look into this issue at the earliest.
@lhasadreams fyi