provider/openstack: Handle Deleted Resources in Floating IP Association

[jtopjian]

This commit modifies the openstack_compute_floatingip_associate_v2 resource
to handle cases where the floating IP or instance were deleted outside of
Terraform.

For #14461

[jtopjian]

==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 01:37:09 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/openstack -v -run=TestAccComputeV2FloatingIPAssociate -timeout 120m
=== RUN   TestAccComputeV2FloatingIPAssociate_importBasic
--- PASS: TestAccComputeV2FloatingIPAssociate_importBasic (94.97s)
=== RUN   TestAccComputeV2FloatingIPAssociate_basic
--- PASS: TestAccComputeV2FloatingIPAssociate_basic (85.95s)
=== RUN   TestAccComputeV2FloatingIPAssociate_fixedIP
--- PASS: TestAccComputeV2FloatingIPAssociate_fixedIP (82.33s)
=== RUN   TestAccComputeV2FloatingIPAssociate_attachToFirstNetwork
--- PASS: TestAccComputeV2FloatingIPAssociate_attachToFirstNetwork (93.30s)
=== RUN   TestAccComputeV2FloatingIPAssociate_attachToSecondNetwork
--- PASS: TestAccComputeV2FloatingIPAssociate_attachToSecondNetwork (132.66s)
=== RUN   TestAccComputeV2FloatingIPAssociate_attachNew
--- PASS: TestAccComputeV2FloatingIPAssociate_attachNew (129.15s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/openstack      618.396s

[stack72]

LGTM :)

[ghost]

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