Fix a typo in the comment and getting started guide, use consistent lower case provider id

I just noticed a typo while working on the "registering a third party provider" Libcloud docs (https://ci.apache.org/projects/libcloud/docs/other/registering-a-third-party-driver.html).

Besides that I also changed all the examples and documentation to use lowercase provider id (`stratuslab` instead of `STRATUSLAB`), because currently some places used uppercase version and other lowercase version.