workload validation before registering elektron. (#19)

Added a utility to help with validating structs. This utility accepts
validators and runs them. If any of the validators fail, then the
error is wrapped with a given base message and returned.

Added validators for checking different attributes of a task
definition.

Added test code to test task validators.

Retrofitted scheduler.go to just log the task validation error and
terminate. If task validation does not report any error, then the
tasks are provided to the scheduler and elektron registers itself
with Mesos.

def/task.go

@@ -20,6 +20,7 @@ package def
 
 import (
 	"encoding/json"
+	"github.com/spdfg/elektron/utilities/validation"
 	"os"
 
 	mesos "github.com/mesos/mesos-go/api/v0/mesosproto"
@@ -55,6 +56,18 @@ func TasksFromJSON(uri string) ([]Task, error) {
 		return nil, errors.Wrap(err, "Error unmarshalling")
 	}
 
+	// Validating task definitions.
+	for _, task := range tasks {
+		err := validation.Validate("invalid task definition",
+			ValidatorForTask(task,
+				withNameValidator(),
+				withImageValidator(),
+				withResourceValidator()))
+		if err != nil {
+			return tasks, err
+		}
+	}
+
 	initTaskResourceRequirements(tasks)
 	return tasks, nil
 }