amazon createServer: use pluralized keys for SecurityGroups/SecurityGroupIds
amazon createServer: use pluralized keys for SecurityGroups/SecurityGroupIds

Fixes #432

apla/pkgcloud,DonSchenck/pkgcloud,mcanthony/pkgcloud,hacor/pkgcloud,pkgcloud/pkgcloud,andrewsomething/pkgcloud,Phanatic/pkgcloud,dionjwa/pkgcloud,Phanatic/pkgcloud,Hopebaytech/pkgcloud,bunglestink/pkgcloud,temka1234/pkgcloud