Add ability to use bean names for jobs and steps

[fmbenhassine]

As of v5.2, both AbstractJob and AbstractStep implement BeanNameAware. So it could be possible to use the bean name as a default job/step name in order to avoid duplication:

@Bean
public Job job(JobRepository jobRepository, Step step) {
--	return new JobBuilder("job", jobRepository)
++	return new JobBuilder(jobRepository)
                           .start(step)
                           .build();
}

Obviously, the current constructor that takes the job/step name should remain to be able to specify a different name if needed.

cc @joshlong