Add a --no-sign-request argument

If this argument is provided then we will _not_ sign
any requests to AWS services.

This depends on https://github.com/boto/botocore/pull/333
Fix completer test with new --no-sign-request arg
Update changelog with no sign request option
Merge branch 'no-sign-request' into develop

* no-sign-request:
  Update changelog with no sign request option
  Fix completer test with new --no-sign-request arg
  Add a --no-sign-request argument