Dropped support for `DRONE_*` environment variables in pipeline steps. Pipeline meta-data can be accessed with `CI_*` variables.
- `CI_*` prefix replaces `DRONE_*`
- `CI` value is now `woodpecker`
- `DRONE=true` has been removed
test for a bug where pipelines that use `depends_on` were not built in multi-pipeline builds. The problem is that pipelines names keep a leading `'/'` when the pipeline path does not have a trailing `'/'`.
* move api code to server/api
* move grpc server for agent communication to server/grpc
* move server.Config to server/config.go as it is used by both server/api and server/grpc
* move shared code used by server/api and server/grpc to server/shared
