138711: roachprod: refactor gcp environment variable initialization r=vidit-bhat,srosenberg a=shailendra-patel

Moved the initialization of default variables related to the GCP Project and DNS into the GCP provider Init function instead of initializing them during package initialization. Setting default variables as global variables caused issues when environment variables are modified before calling Init.

Epic: none

Release note: None

Co-authored-by: Shailendra Patel <[email protected]>