Add NAT router configuration also for Ops Manager

[jsoriano]

Terraform configuration creates an storage bucket for the Ops Manager,
but Ops Manager cannot connect to google cloud if it doesn't have a NAT
router.

These errors appear if the management subnet doesn't have a NAT router config:

Task 4 | 18:42:58 | Error: Failed to create GCS object, code 1, output: 'Get https://www.googleapis.com/storage/v1/b/project-id-pcf-ops-manager-36547?alt=json&prettyPrint=false&projection=full: oauth2: cannot fetch token: Post https://oauth2.googleapis.com/token: dial tcp 216.58.205.42:443: i/o timeout
', error: '2021/01/11 18:42:58 performing operation put: Get https://www.googleapis.com/storage/v1/b/project-id-pcf-ops-manager-36547?alt=json&prettyPrint=false&projection=full: oauth2: cannot fetch token: Post https://oauth2.googleapis.com/token: dial tcp 216.58.205.42:443: i/o timeout

[bgandon]

I've run into the same issue, and cherry-picking the patch helped me a lot, thank you!
Maintainers, it would be great to merge this PR 👍

[jaristiz]

Looks good to me.
Will schedule it to be included on the next release

[bgandon]

Nice, thank you!