Improve SCM Integration

Screwdriver is a set of services facilitating flawless workflow implementations for continuous delivery pipelines. It provides integrations to interact with various SCMs (Source Control Management Systems) and deliver the project source codes to production. This project deals primarily to improve the integrations of various SCMs (Source Control Management Systems) in Screwdriver.cd. When developing pipelines for enterprise scale projects, it is imperative that a seamless interface is provided for interacting with private repositories in SCMs. Apart from this, enterprise projects feature pipelines built from multiple dependencies which have active development going on simultaneously. Therefore, it is important to ascertain that the CI/CD tooling works efficiently with these external SCM repositories. Hence, the two key areas where it will be improved are introduction of deployment keys for seamless handling of private repositories and triggering of builds from external SCM repositories.