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.

utilities/validation/validate.go

@@ -0,0 +1,40 @@
+// Copyright (C) 2018 spdfg
+//
+// This file is part of Elektron.
+//
+// Elektron is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Elektron is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Elektron.  If not, see <http://www.gnu.org/licenses/>.
+//
+
+// Package validation contains utilities to help run validators.
+package validation
+
+import "github.com/pkg/errors"
+
+// Validator is a function that performs some sort of validation.
+// To keep things generic, this function does not accept any arguments.
+// In practice, a validator could be a closure.
+type Validator func() error
+
+// Validate a list of validators.
+// If validation fails, then wrap the returned error with the given base
+// error message.
+func Validate(baseErrMsg string, validators ...Validator) error {
+	for _, v := range validators {
+		if err := v(); err != nil {
+			return errors.Wrap(err, baseErrMsg)
+		}
+	}
+
+	return nil
+}