Document public methods with Rdoc

Document the public API methods using Rdoc.

Note I have not documented `NetLaunch#new` as there was nothing I could
add that would not repeat the documentation for `NetLaunch#run` and I
think it's simple enough so as to be self-explanatory.

Also, YARD (though not Rdoc) will automatically document it as:

> A new instance of NetLaunch.
Document public method parameters
Merge pull request #41 from gds-operations/73962736-document-api

[#73962736] Document public methods with Rdoc
[#66147100] remove the cli option for mocking

Remove the cli option to enable fog mocking. None of other tools have one and
people use and environment variable to use it anyway.

Also move the enabling of fog mocking via an environment variable to the spec_helper
instead of in the class.