Support Jenkins as a execution tool for pipeline

[BrianJakovich]

Enable a user to leverage Jenkins to execute stages of a pipeline instead of CodeBuild.

Features:

• Jenkins can be the execution tool in place of CodeBuild and leverage CodePipeline as the orchestrator
• Jenkins can be the execution tool in place of CodeBuild and also be the orchestration tool in place of CodePipeline
• Could support both an existing Jenkins environment and allow users to create a new Jenkins environment

Work involved:

• Create CloudFormation template for launching up a Jenkins cluster
• Create a "provider" to support Jenkins? @cplee

[jeffb4]

A variant of this that has been discussed is to have mu generate a Jenkinsfile in a repo to reify a pipeline, and then to (future work) add the job/repo to an existing Jenkins master server via an API call.

Choices:

• specify in mu.yml that a pipeline should be of type jenkinsfile or type codepipeline, and change behavior of existing pipeline subcommands

• create a new mu subcommand for creating a Jenkinsfile (and later for adding job via API)

Some architecture decisions by @cplee needed