(maint) vpc_only_account? logic improvements
(maint) vpc_only_account? logic improvements

 - Clean up code to provide a more self-descriptive mechanism
   for finding VPC only accounts.

   With clasic accounts, account_types is ["EC2", "VPC"] while
   a VPC only account has the values ["VPC"].  It's possible
   that there could be other / legacy account types that have
   a different list of supported-platforms, or that this
   changes in the future.

gregohardy/puppetlabs-aws,BashtonLtd/puppetlabs-aws,puppetlabs/puppetlabs-aws,zleswomp/puppetlabs-aws,DavidS/puppetlabs-aws,DavidS/puppetlabs-aws,lattwood/puppetlabs-aws,garethr/puppetlabs-aws,marksyme/puppetlabs-aws,akozichev/puppetlabs-aws,bmjen/puppetlabs-aws,Yelp/puppetlabs-aws,HelenCampbell/puppetlabs-aws,zreichert/puppetlabs-aws,lattwood/puppetlabs-aws,tamsky/puppetlabs-aws,akozichev/puppetlabs-aws,zreichert/puppetlabs-aws,marksyme/puppetlabs-aws,zleswomp/puppetlabs-aws,daveseff/puppetlabs-aws,puppetlabs/puppetlabs-aws,sarguru/puppetlabs-aws,rfletcher/puppet-aws,HelenCampbell/puppetlabs-aws,sarguru/puppetlabs-aws,jhoblitt/puppetlabs-aws,CarmenAPuccio/puppetlabs-aws,hundredacres/puppetlabs-aws,benpillet/puppetlabs-aws,sarguru/puppetlabs-aws,benpillet/puppetlabs-aws,tamsky/puppetlabs-aws,CarmenAPuccio/puppetlabs-aws,rfletcher/puppet-aws,Yelp/puppetlabs-aws,zvymazal/puppetlabs-aws,BashtonLtd/puppetlabs-aws,daveseff/puppetlabs-aws,akozichev/puppetlabs-aws,bmjen/puppetlabs-aws,hundredacres/puppetlabs-aws,jhoblitt/puppetlabs-aws,zreichert/puppetlabs-aws,BashtonLtd/puppetlabs-aws,garethr/puppetlabs-aws,gregohardy/puppetlabs-aws,Yelp/puppetlabs-aws,zvymazal/puppetlabs-aws,bmjen/puppetlabs-aws