Rename ':first_boot_attributes' option

I'm using knife-solo along with knife-ec2 using the POC from @tmatilai.  When running `knife ec2 server create --solo --json-attributes X` the attributes passed in were not getting assigned to the `:first_boot_attributes` option but rather the `:json_attributes` option defined in knife-ec2.

I changed the option name to match the option from knife-ec2.  This works regardless of whether you integrate with knife-ec2 or not.

The other options defined here (`:run_list`, `:chef_node_name`) already match those in knife-ec2 so there is no conflict when using those.