Read named ports changes properly in google_compute_instance_group

[rosbo]

Fixed #170

Set the named ports when reading the current state of an instance group.
Removed the call to the read method in the update method before performing the actual update, this overrides the changes to the named ports field with my fix.
In order to handle the case where referenced instances have been removed earlier in the run, instead of doing the pre-read, don't fail if the InstanceGroups#RemoveInstances method returns a 404 (Not Found). If the instances have already been removed, this is the state we want to be in anyway.
Fixed a few typos
Updated tests to cover the case where referenced instances have been removed earlier in the run.

/cc @danawillow

[danawillow]

Looks good, congrats on your first contribution!

make testacc TEST=./google TESTARGS='-run=TestAccComputeInstanceGroup'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccComputeInstanceGroup -timeout 120m
=== RUN   TestAccComputeInstanceGroup_basic
--- PASS: TestAccComputeInstanceGroup_basic (117.66s)
=== RUN   TestAccComputeInstanceGroup_update
--- PASS: TestAccComputeInstanceGroup_update (162.80s)
=== RUN   TestAccComputeInstanceGroup_outOfOrderInstances
--- PASS: TestAccComputeInstanceGroup_outOfOrderInstances (113.18s)
=== RUN   TestAccComputeInstanceGroup_network
--- PASS: TestAccComputeInstanceGroup_network (150.13s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	543.889s

[tomwilkie]

Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 [email protected]. Thanks!