Add better tests for Bosh::Stemcell::Aws

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Remove some deprecation warnings

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Clean up stemcell_publisher_spec specs

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Build hvm light aws stemcell in addition to pv

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Partial rewrite of definition_spec

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Updated stemcell README

Signed-off-by: Andrei Dinin <adinin@pivotal.io>