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.
Merge pull request #193 from benjic/cloudconfig_variables

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