Write entire backend service resource when updating

[selmanj]

Also update the default value for connection_draining_timeout_sec to
match the default used by GCP

Fixes #348

[rosbo]

TF_ACC=1 go test ./google -v -run TestAccComputeBackendService_ -timeout 120m
=== RUN TestAccComputeBackendService_importBasic
--- PASS: TestAccComputeBackendService_importBasic (45.14s)
=== RUN TestAccComputeBackendService_basic
--- PASS: TestAccComputeBackendService_basic (67.49s)
=== RUN TestAccComputeBackendService_withBackend
--- PASS: TestAccComputeBackendService_withBackend (92.82s)
=== RUN TestAccComputeBackendService_withBackendAndUpdate
--- PASS: TestAccComputeBackendService_withBackendAndUpdate (104.59s)
=== RUN TestAccComputeBackendService_updatePreservesOptionalParameters
--- PASS: TestAccComputeBackendService_updatePreservesOptionalParameters (56.53s)
=== RUN TestAccComputeBackendService_withConnectionDraining
--- PASS: TestAccComputeBackendService_withConnectionDraining (44.51s)
=== RUN TestAccComputeBackendService_withConnectionDrainingAndUpdate
--- PASS: TestAccComputeBackendService_withConnectionDrainingAndUpdate (56.28s)
=== RUN TestAccComputeBackendService_withHttpsHealthCheck
--- PASS: TestAccComputeBackendService_withHttpsHealthCheck (44.11s)
=== RUN TestAccComputeBackendService_withCDNEnabled
--- PASS: TestAccComputeBackendService_withCDNEnabled (44.33s)
=== RUN TestAccComputeBackendService_withSessionAffinity
--- PASS: TestAccComputeBackendService_withSessionAffinity (56.59s)
PASS
ok github.com/terraform-providers/terraform-provider-google/google 612.528s

[ghost]

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