Feature Request: Skip docker push when deploying

### Describe your idea/feature/enhancement

User story: As a developer, I would like to have the option to bring my own image tag to `sam deploy` and skip the `docker push` step.

Backstory: My organization uses Concourse CI as our CI/CD tooling. In our CI pipelines, we would rather use Concourse's built in docker image build mechanism rather than have to run `sam build`. Likewise when running `sam package` or `sam deploy` it would be good to skip the `docker push` step and provide an image tag to use in the cloudformation instead.

Another goal of this is to allow for a promotion based CD workflow for different environments, using semver tagged docker images. For docker images, this would also decouple `sam package`/`sam deploy` from `sam build` and allow for more flexibility in CI systems.

### Proposal

add a flag to `sam package`/`sam deploy` to skip the docker tag and push such as `--skip-image-push` and allow the image tag to be overridden as a parameter or provide some way to override the function metadata. That part is less clear to me so open to suggestions there.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)