Automatic merge from submit-queue (batch tested with PRs 48698, 48712, 48516, 48734, 48735)

GCE: Allow empty NETWORK_PROJECT_ID env var

Changes:
1. Adds `GCE_API_ENDPOINT` logic to container-linux as it was added to GCI in #47881.
1. Apply `NETWORK_PROJECT_ID` value to gce.conf only if the env var is set.

/sig network
/area platform/gce

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