Add gscloud-kubernetes-releases

[bkircher]

Implement a subcommand gscloud kubernetes versions that essentially prints the available version slugs for Kubernetes from the objects/paas/service_templates endpoint.

Example:

$ curl -s -H 'Content-Type: application/json' \
  https://api.gridscale.dev/objects/paas/service_templates \
  -H "X-Auth-UserID: $userid" \
  -H "X-Auth-Token: $apitoken \
  | jq '.paas_service_templates[].version'

"1.16.15-gs2"
"1.17.14-gs1"
"1.18.12-gs1"
"1.19.4-gs1"

[bkircher]

Of course this should be kubernetes releases as in #118

[bkircher]

Example output:

$ gscloud kubernetes releases 
RELEASES  
1.16      
1.17      
1.18      
1.19