Make config validation in common way

Each engine and provider can have property CONFIG_SCHEMA. Validation can be done in base constructor by calling validate method. Validate method can be overrided if needed.

 class EngineFactory(object):
  def __init__(self, task, config):
   self.validate(config)
   self.task = task
   self.config = config

 def validate(config):
   jsonschema.validate(self.CONFIG_SCHEMA, config)
   if somethingwrong:
    raise exceptions.ConfigValidationError()

 class SomeEngine(EngineFactory):
  CONFIG_SCHEMA = {'some': 'schema'}
  def __init__(self, task, config):
   super (SomeEngine, self).__init__()
   do_own_init()

 class SomeEngineWithComplexValidation(EngineFactory):
  def validate(config):
   do_complex_validation()

The same for Provider

Also rally.exceptions.ConfigValidationError exception should be added.