Allow updating google_container_cluster.monitoring_service

[danawillow]

Fixes #308.
Part of #474.

Original PR is #309, this update also adds tests.

[danawillow]

$ make testacc TEST=./google TESTARGS='-run=TestAccContainerCluster_withMonitoring'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccContainerCluster_withMonitoring -timeout 120m
=== RUN   TestAccContainerCluster_withMonitoring
--- PASS: TestAccContainerCluster_withMonitoring (594.71s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	594.933s

[paddycarver]

LGTM

[ghost]

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