serverless.yml contains region more than once

[vboctor]

When using serverless-slack I had an issue where I changed the region and it didn't work. Turned out that serverless.yml references the region in multiple locations and I didn't update all of them. It would be good if the region is specified in one location and re-used from there, so it is easy to update it without causing issues.

[vboctor]

It would be good to have deploy fail until this is set or at least have it as an explicit setup step in readme file.