Add flags for backend options

[anbraten]

We added some backend options like WOODPECKER_BACKEND_SSH_ADDRESS in #893 and #861 which are accessed by os.Getenv directly. As the backend can be executed by the agent and by the cli exec command those settings need to be available for both and should be listed in the documentations of both systems.

To properly use them with urfave cli we should think about placing a flags.go file into each backend package and include this in the cli exec and agent options. This would allow us to see those options in the help of both tools and auto document them.