docs(quick-start): Clarified use of fields in cloud config

Updated the language to illustrate that fields in a cloud config is not
supported in all environments. This is expressed explicitly in PXE and
install to disk pages. The quick start lacked this information and is
inconsistent.
docs(quick-start) Enumerated supported platforms

Following suggestion a list of platforms that *do* support cloud config variables. In addition minor mark up formatting is added.
feat(datasource) PoC implementation for the CloudSigma server context
feat(*): Add the server context datasource in the main flags
refactor: Add ExtractIPs method to the Datasource interface
test: Add tests and simplify the userdata crunching
refactor: Fetch byte arrays from the serial console

That allows me use native IP guessing
feat(datasource/server_context): Put cepgo in the source tree
refactor: Remove leftover ExtractIPs method in remoteFile