load-balancer setup for GCP

[yeoldegrove]

This PR is part of a series of PRs to enable a load-balancer setup for GCP (Netweaver, DRBD, HANA [already existing]). The formulas for HANA, Netweaver, DRBD are streamlined to have the same logic to create cluster resources. PRs:

• https://github.com/SUSE/saphanabootstrap-formula/pull/137

• https://github.com/SUSE/sapnwbootstrap-formula/pull/91

• https://github.com/SUSE/drbd-formula/pull/28

• https://github.com/SUSE/ha-sap-terraform-deployments/pull/793

• gcp: add option to enable load-balancer for drbd -> new default as it is already for HANA

  • add common_variables['drbd'] and cleanup unneeded variables
  • old route option is still available

• gcp: add option to enable load-balancer for netweaver -> new default as it is already for HANA

  • old route option is still available

fixes https://github.com/SUSE/ha-sap-terraform-deployments/issues/783