Add k8s cluster resource (#140)

Add a new k8s resource to TF provider. See #120.

What changes:

- Add new tf resource "gridscale_k8s".
- Add "gridscale_k8s" resource's docs.

What it does:

- Manage k8s cluster resources in gridscale.
- Validate gsk (gridscale k8s cluster) parameters. Much better than what we had before.
- Allow user to input k8s_release (e.g. 1.19), instead of inputting service_template_uuid. Optionally, the values of `release` can be retrieved via gscloud (since 0.10 release).

gridscale/provider.go

@@ -77,6 +77,7 @@ func Provider() terraform.ResourceProvider {
 			"gridscale_snapshotschedule":               resourceGridscaleStorageSnapshotSchedule(),
 			"gridscale_backupschedule":                 resourceGridscaleStorageBackupSchedule(),
 			"gridscale_paas":                           resourceGridscalePaaS(),
+			"gridscale_k8s":                            resourceGridscaleK8s(),
 			"gridscale_paas_securityzone":              resourceGridscalePaaSSecurityZone(),
 			"gridscale_postgresql":                     resourceGridscalePostgreSQL(),
 			"gridscale_object_storage_accesskey":       resourceGridscaleObjectStorage(),