[API] [Workbench] set a default config.log_level
log_level isn't set by default, triggering the following deprecation warning:
DEPRECATION WARNING: You did not specify a `log_level` in `production.rb`. Currently, the default value for `log_level` is `:info` for the production environment and `:debug` in all other environments. In Rails 5 the default value will be unified to `:debug` across all environments. To preserve the current setting, add the following line to your `production.rb`: config.log_level = :info
Arvados out-of-the-box should come with a config.log_level set. My suggestion is to make it ":info"