Rename ':first_boot_attributes' option

When using knife-solo with knife-ec2 the --json-attributes option
would be assigned to ':json_attributes' rather than
':first_boot_attributes'.  This would prevent json attributes being
passed to knife-solo when running 'knife ec2 server create --solo'

Renamed the option here to match the option from knife-ec2.
Fallback to :first_boot_attributes if :json_attributes not specified

`knife bootstrap` uses :first_boot_attributes and knife-ec2 copies the
value there. So if knife solo is injected directly to Knife::Bootstrap
and Bootstrap class initialized by e.g. knife-ec2 we need to support
that option, too.
Merge branch 'russellcardullo/json-options'

Closes PR #166.