fix: Allow setting maximum-backup-rate on create postgres step

[amontalban]

This PR allows configuring the maximum-backup-rate setting when running pg_autoctl create postgres.

Fixes #780

[ghost]

All CLA requirements met.

[DimCitus]

Thanks for the PR @amontalban ! Looks good to me, mostly, apart from a very minor detail that we still want to fix before merging. Could you add a commit to that effect in your branch?

[DimCitus]

It's now failing the make indent checks. If you can, check out our https://github.com/citusdata/pg_auto_failover/blob/master/CONTRIBUTING.md guidelines and then do make indent and push the new version of your PR, otherwise I'm going to look into doing that myself (but I'm not sure if I'm allowed to push to your branch).

[amontalban]

It's now failing the make indent checks. If you can, check out our https://github.com/citusdata/pg_auto_failover/blob/master/CONTRIBUTING.md guidelines and then do make indent and push the new version of your PR, otherwise I'm going to look into doing that myself (but I'm not sure if I'm allowed to push to your branch).

Sorry @DimCitus, I tried that before in my macOS but the Makefile was failing. I got it working and I think it should pass now.

[DimCitus]

Thanks for your work @amontalban ; both the bug report and now the bug fix, and welcome to the pg_auto_failover contributors team!