Add gke node taints

[danawillow]

Fixes #680

[emilymye]

LGTM

$ make testacc TEST=./google TESTARGS="-run='TestAccContainerCluster_withNodeConfigTaints'"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run='TestAccContainerCluster_withNodeConfigTaints' -timeout 120m
=== RUN   TestAccContainerCluster_withNodeConfigTaints
=== PAUSE TestAccContainerCluster_withNodeConfigTaints
=== CONT  TestAccContainerCluster_withNodeConfigTaints
--- PASS: TestAccContainerCluster_withNodeConfigTaints (214.99s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	215.175s

$ $ make testacc TEST=./google TESTARGS="-run='TestAccContainerNodePool_withNodeConfigTaints'"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run='TestAccContainerNodePool_withNodeConfigTaints' -timeout 120m
=== RUN   TestAccContainerNodePool_withNodeConfigTaints
=== PAUSE TestAccContainerNodePool_withNodeConfigTaints
=== CONT  TestAccContainerNodePool_withNodeConfigTaints
^[^[--- PASS: TestAccContainerNodePool_withNodeConfigTaints (416.72s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	416.895s

[AndreaGiardini]

Do we need to docs for this feature? I can't find it in the official documentation https://www.terraform.io/docs/providers/google/r/container_node_pool.html

[danawillow]

It's at https://www.terraform.io/docs/providers/google/r/container_cluster.html#taint, since it's part of the node_config, which is common between the cluster and the node pool.

[ghost]

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