Return channel on which routine will listen instead of allowing to specify own anonymous function.

It's possible that we will block on Notify anonymous function, when the select loop a few lines below gets finished.
It will happen, because we use unbuffered channel, but don't have a possibility to consume it.
Thus this will block trace sending routing.
Trace sending routine will not finish, thus it will block trace.Success/Fail resulting in ability to cleanup a executor and releasing a machine.

This also renames BuildAbort to SystemInterrupt which is more accurate and makes a timeout a BuildError instead of SystemError.

We check for presence of gitlab-runner and execute a command then.

Kamil Trzciński authored 8 years ago and Tomasz Maczukin committed 8 years ago

- Introduce BuildError to generate soft failure on build error and generate hard failure on system error
- Use WithField instead of pushing data as arguments of most of logging functions
- Introduce BuildLogger which removes logging functionality from AbstractExecutor
- Use BuildLogger and write notice message as early as possible
- Remove finish notice/error from AbstractExecutor
- Sending and appending build trace is now debug message by default

- Rename `ShellScript` to `ShellConfiguration` and remove all generated scripts
- Introduce `ShellScriptType` with different types of scripts
- Introduce `GetConfiguration` and `GenerateScript` for `Shell` interface
- Change `ShellScript` to `Shell` of `Executor`
- Remove `GenerateScript` from shells, instead provide a generic method with accepts `ShellScriptType`
- Introduce `executeShellScript` for `Build`, method which is responsible for generating and executing a shell script

Fixes #1114

This commit gets rid of all indirect access to config variables and uses a new `toml` features: `omit empty` and `omit zero`

- Introduces ExecutorProvider
- Renames some private methods of AbstractExecutor

- use NormalShell only when executing builds in docker container - it allows to preserve all environment variables set in Dockerfile
- use LoginShell in all other cases