text: Title: should check both 'http_proxy' and 'HTTP_PROXY' when get proxy info from environment variables
  Body: ## SUMMARY
  
  I just faced an embarrassed problem that command `st2 action list` keeps return 503. 
  
  I tried `st2 --debug action list` and the result in line  `HTTP_PROXY`(**uppercase**) was empty. This mislead me that the proxy info was set(which is not).
  
  I finally found that the root cause is a **lowercase** `http_proxy` env variable was set in the system.
  
  So I think it maybe more friendly to check both 'http_proxy' and 'HTTP_PROXY' to get proxy info when using st2 command with `--debug` option.
  
  related code:
  https://github.com/StackStorm/st2/blob/5c4e5f8e93c7ed83c4aa4d196085c2912ae7b285/st2client/st2client/base.py#L404
  
  
  ### STACKSTORM VERSION
  
  3.3
  
  ##### OS, environment, install method
  centos 7.6, custom install
  
  ## Steps to reproduce the problem
  
  1) set **lowercase** proxy environment variable, eg. `export http_proxy='127.0.0.1:8888'`
  2) run `st2 --debug action list`
  
  ## Expected Results
  
  proxy info should be printed
  
  ## Actual Results
  
  proxy info did not show, value of HTTP_PROXY is empty.
  
  Thanks!
  