Makes the stemcell resource configurable for bosh-integration
Makes the stemcell resource configurable for bosh-integration

This allows the bosh-integration job to use a non bosh-io-stemcell typed
resource so we can use a custom stemcell when running this job out of
aws china

[#104220528](https://www.pivotaltracker.com/story/show/104220528)

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release