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Fix master regex when running multiple clusters

**What this PR does / why we need it**:
I'm running two Kubernetes clusters on GCE. One for production and one for staging. The instance prefix I use for production is `kubernetes` and for staging it's `staging-kubernetes`. This caused a problem when running `kube-up.sh` for production because when it tries to find all instances which match `kubernetes(-...)?` it finds both the production and staging instances. This probably results in multiple problems, but the most noticeable one for me was that `INITIAL_ETCD_CLUSTER` was incorrect and so etcd wouldn't start up correctly so the api server doesn't start up correctly so nothing else starts up. I tested this manually and it seems to work for me, but I didn't write an automated test.

**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
Fixes bug finding master replicas in GCE when running multiple Kubernetes clusters
```