Added handling of fixnum values for shell provisioner options both as the value
of `args` itself and as an element inside of an `args` array. Added validation
around passing in arrays for arguments. Just knowing that it is an array is not
enough, because arrays can contain any other type.
Added unit test for passing a single fixnum as an argument to the shell
provisioner
Removed excess whitespace
Merge pull request #2982 from ryanuber/shell_fixnum_args

provisioner/shell: shell provisioner fixnum args
provisioners/shell: clean up fixnum handling
detect latest version of coreos
more robust coreos checking
Merge pull request #2985 from eugeneware/latest_coreos_support

guests/coreos: detect latest version.