Use UPPERCASED beaker environment variable

Follow-up of:
voxpupuli/beaker-rspec#117

The old names or still supported but for consistency change them to the
new ones.

Author: smortex

_docs/how_to_run_tests.md

@@ -48,7 +48,7 @@ To limit test execution to a certain os or os release you can set the environmen
 
 ## Acceptance tests
 
-`BEAKER_setfile=centos7-64 bundle exec rake beaker`
+`BEAKER_SETFILE=centos7-64 bundle exec rake beaker`
 
 How to run the acceptance tests is described more in detail on this page:
 [voxpupuli-acceptance](https://github.com/voxpupuli/voxpupuli-acceptance/#running-tests)