[cloudformation] fix disable_rollback typos
[docs/cloudformation] new doc describing common cloudformation parameters
[docs/cloudformation] formatting fixes
[cloudformation] rename platform parameter to machine image

Because Signifyd's primary application is known internally as Platform,
it seems wise to disambiguate the parameter name from that application.
[cloudformation] add ssh key id parameter
[docs/cloudformation] document the platform template
[docs/cloudformation] fix nexus/platform copy pasta
Merge pull request #8 from signifyd/docs/cloudformation

Additional CloudFormation documentation, fix CLOUDFORMATION_DISABLE_ROLLBACK