update CONTRIBUTING with information about beaker

beaker has replaced our rspec-system tests. These new tests are run
differently. Before messing with our Rakefile, a good start is to just
document it.
Merge pull request #533 from igalic/contributing-beaker

update CONTRIBUTING with information about beaker
Support environment variable control for CustomLog
Rename var-name: access_log_environment_variable to access_log_env_var