Expose the retention variable in apigee-x-core

[carlosmscabral]

From https://github.com/GoogleCloudPlatform/cloud-foundation-fabric/blob/master/modules/apigee/variables.tf , please expose the variable RETENTION. Specially useful for IaaC - if we need to rollback something in the Org, default is waiting for 7 days. Being able to specify minimum retention is useful.

[danistrebel]

Thanks for the suggestion, Feel free to open a PR for this.