Simplify Run syntax

[nleiva]

Follow up from #37 (comment)

The metadata or descriptions are not too relevant for the actual execution.

For pre-defined tasks (like SSHClose) I propose we provide a default description (the user can change it). The user can also create its own tasks:

type SSHOpen struct {
	description string
}

type SSHClose struct {
	description string
}

type MyTask struct {
	description string
}

func NewSSHOpen() SSHOpen {
	return SSHOpen{
		description: "Open SSH connection",
	}

}

func NewSSHClose() SSHClose {
	return SSHClose{
		description: "Close SSH connection",
	}

}

The goal is the user program at very high level looks something like (MyTask in the same connection package for simplicity in this example):

func main() {
	...
	sshOpen := connection.NewSSHOpen()
	sshClose := connection.NewSSHClose()
	myTask := connection.MyTask{description: "Something else"}
	
	gr.RunSync(sshOpen)
	defer gr.RunSync(sshClose)
	gr.RunSync(myTask)

	fmt.Println("Doing this:", sshOpen.description, myTask.description, sshClose.description)
	...

}

The user creates tasks that are then executed in single lines.

[dbarrosop]

Not sure if this is the right way but I wanted to get rid of the JobParameters object and as a side effect I had to reconsider the "title". For now, I have moved it to the RenderResult as it's where it's being consumed but we may still want to be able to attach some metadata to the jobs:

b22aa1c