Update --resources flag

[Ethan-Arrowood]

Description

Closes #14 Ref https://github.com/Azure/platform-chaos/issues/31 Removes conditional block for comma separated list of resources. Adds documentation to show user exactly how they should pass in multiple resources.

Checklist for success

• [x] This change isn't already implemented by a pending pull request
• [x] This change is a child commit of a reasonably recent commit in the master branch
  • Requests need not be a single commit, but should be a linear sequence of commits (i.e. no merge commits in your PR)
• [x] All tests pass locally (see the testing section for more info)
• [x] All New behaviors or major functional changes have tests

[bengreenier]

@Ethan-Arrowood - please delete the source branch if/when you're ready to do so!