Add fix for bug introduced in Terraform v0.11.8

[robmorgan]

This PR adds a fix for a bug introduced in Terraform v0.11.8. For more information please see: hashicorp/terraform-provider-google#2067.

Steps to Reproduce

1. Install Terraform v0.11.8.
2. Checkout the master branch.
3. Run the automated tests from the test directory: go test -v -timeout 60m -run TestConsulClusterWithUbuntuImage.

Result

TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100: Error: Error refreshing state: 2 error(s) occurred:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100: * module.consul_clients.google_compute_instance_template.consul_server_public: 1 error(s) occurred:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100: * module.consul_clients.google_compute_instance_template.consul_server_public: google_compute_instance_template.consul_server_public: error flattening disks: Error getting relative path for source image: String was not a self link: global/images/consul-2018-10-02-010549
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100: * module.consul_servers.google_compute_instance_template.consul_server_public: 1 error(s) occurred:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100: * module.consul_servers.google_compute_instance_template.consul_server_public: google_compute_instance_template.consul_server_public: error flattening disks: Error getting relative path for source image: String was not a self link: global/images/consul-2018-10-02-010549
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 command.go:100:
TestConsulClusterWithUbuntuImage 2018-10-02T15:15:24+02:00 retry.go:77: Returning due to fatal error: FatalError{Underlying: exit status 1}
--- FAIL: TestConsulClusterWithUbuntuImage (583.07s)
    apply.go:13: FatalError{Underlying: exit status 1}
    destroy.go:11: FatalError{Underlying: exit status 1}
FAIL
exit status 1
FAIL    github.com/robmorgan/terraform-google-consul/test       583.101s

Expected Result

PASS
ok      github.com/robmorgan/terraform-google-consul/test       836.258s

[Etiene]

LGTM!

[josh-padnick]

Nice fix! 👍