Add preview mode customization

This hides certain commands as preview services.  A user
has to opt into this service via the config file if they want
to use this service.
Add _UNDOCUMENTED class attr to preview commands
Add test for provider help
Update README to catch casing changes
Fix typo
Merge branch 'preview-mode' into develop

* preview-mode:
  Add test for provider help
  Add _UNDOCUMENTED class attr to preview commands
  Add preview mode customization
Adding customizations for ec2 bundle-instance to allow users to enter scalar params and to generate a polciy if not provided and to sign the policy.
Simplifying the parameter objects.  Fixing some doc strings.
Adding customizations for ec2 bundle-instance to allow users to enter scalar params and to generate a polciy if not provided and to sign the policy.
Simplifying the parameter objects.  Fixing some doc strings.
Fix Py3 encoding issue.
Fix another Py3 encoding problem.
more tweaks to tests.
Moving _build_storage into BundleArgument class.
Resolving merge conflicts.