Automatic merge from submit-queue (batch tested with PRs 67736, 68123, 68138). If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

Added support to get clusters in gce cloud provider.

**What this PR does / why we need it**:
Implemented the call to get all cluster objects in a zone for a project.
Also added code to allow the container api to be set in the gce.conf
file.

**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 #

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```