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[GCE] Instance comparable host path
**What this PR does / why we need it**:
When creating a new TargetPool, insert new instances with the comparable host path instead of the full path, e.g. /zone/%s/instances/%s instead of the full https://www.googleapis.com/compute/v1/projects/... url.
With this change, `createTargetPoolAndHealthCheck` and `updateTargetPool` insert gceInstance paths in a consistent manner.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Release note**:
```release-note
NONE
```
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