migration.yml template expects $DR_BUCKET to be defined but this was not required in px-deploy 4.x

[davidcastellani]

For those of us who have been using px-deploy since before 5.x the migration template was using the built in object storage built into PX. No additional bucket creation was required.

With the move to PX-deploy 5.x this is now a requirement but its not mentioned anywhere. Once DR_BUCKET is defined in something like defaults.yml this template works as expected.

[andrewh1978]

I think we should add a template parameter that lists all the env vars that need to be defined, and error if they are not. @danpaul81 thoughts?